xeth, rpc: implement eth_estimateGas. Closes #930

Closes #880

compiler, cli/js: fix solidity compiler tests

eth, ethdb: lower the amount of open files & improve err messages for db

Closes #880

miner, cmd/geth: miner will not ignored owned account transactions

Miner does not ignore low gas txs from accounts that are owned.

cmd/geth, miner, backend, xeth: Fixed miner threads to be settable

Miner threads are now settable through the admin interface (closes #897)
and specify 0 CPU worker threads when eth_getWork is called (closes #916)

eth/downloader: fix #910, thread safe peers & polishes

Return 32-byte hashes from GetWork

Signature on arbitrary data using the private keys of an account

Ensures that the Get Work results are a consistent length. Closes #917

Conflicts:
	rpc/jeth.go

core, eth, miner: improved tx removal & fatal error on db sync err

* core: Added GasPriceChange event
* eth: When one of the DB flush methods error a fatal error log message
  is given. Hopefully this will prevent corrupted databases from
  occuring.
* miner: remove transactions with low gas price. Closes #906, #903

rpc, xeth: fix #881, gracefully handle offline whisper

ethdb: set open file limit to 128 for all leveldbs

eth: added a cancel method for the downloader