eth/63 fast synchronization algorithm

core, tests: get_hash fix

Make sure that we're fetching the hash from the current chain and not
the canonical chain.

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