core, core/state: move gas tracking out of core/state

The amount of gas available for tx execution was tracked in the
StateObject representing the coinbase account. This commit makes the gas
counter a separate type in package core, which avoids unintended
consequences of intertwining the counter with state logic.

core, eth/filters, miner, xeth: Optimised log filtering

Log filtering is now using a MIPmap like approach where addresses of
logs are added to a mapped bloom bin. The current levels for the MIP are
in ranges of 1.000.000, 500.000, 100.000, 50.000, 1.000. Logs are
therefor filtered in batches of 1.000.

godeps: update ethash following GPU miner merge

all: Add GPU mining, disabled by default

core/state, core, miner: handle missing root error from state.New

Update libsecp256k1, Go wrapper and tests

core, eth, event, miner, xeth: fix event post / subscription race

common, crypto: add ICAP functions

core/vm: copy stack element to prevent overwrites

cmd, core, eth: added official testnet

cmd: properly initialize Olympic for all subcommands

core, core/vm, cmd/evm: remove redundant balance check

crypto: correct sig validation, add missing unit tests of exported functions

console/history respect datadir

core: fix #1848, block receipts db entry for the genesis too

cmd/geth: dev version number

core: fixed head write on block insertion

Due to a rebase this probably got overlooked / ignored. This fixes the
issue of a block insertion never writing the last block.

core, core/vm: refactor

* Renamed ChainManager to BlockChain
* Checkpointing is no longer required and never really properly worked
when the state was corrupted.

* Moved `vm.Transfer` to `core` package and changed execution to call
`env.Transfer` instead of `core.Transfer` directly.
* core/vm: byte code VM moved to jump table instead of switch
* Moved `vm.Transfer` to `core` package and changed execution to call
  `env.Transfer` instead of `core.Transfer` directly.
* Byte code VM now shares the same code as the JITVM
* Renamed Context to Contract
* Changed initialiser of state transition & unexported methods
* Removed the Execution object and refactor `Call`, `CallCode` &
  `Create` in to their own functions instead of being methods.
* Removed the hard dep on the state for the VM. The VM now
  depends on a Database interface returned by the environment. In the
  process the core now depends less on the statedb by usage of the env
* Moved `Log` from package `core/state` to package `core/vm`.

Moved the filtering system from `event` to `eth/filters` package and
removed the `core.Filter` object. The `filters.Filter` object now
requires a `common.Database` rather than a `eth.Backend` and invokes the
`core.GetBlockByX` directly rather than thru a "manager".

eth/downloader: match capabilities when querying idle peers

core: deadlock in chainmanager after posting RemovedTransactionEvent

This PR solves an issue with the chain manager posting a
`RemovedTransactionEvent`, the tx pool will try to
acquire the chainmanager lock which has previously been locked prior to
posting `RemovedTransactionEvent`. This results in a deadlock in the
core.