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

Added a cancel method to the downloader which gracefully shuts down any
active syncing process (hash fetching or block downloading) and resets
the queue and remove any pending blocks.

Issue with the downloader which would stall because of an active ongoing
process when an invalid block was found.

miner, flags: Configurable gas price & log flag change

* --gasprice "<num>" flag
* admin.miner.setGasPrice( <num> )

eth/downloader: prioritize block fetch based on chain position, cap memo...

p2p: more last-minute fixes

This should improve ethereum block propagation times since
we're not not broadcasting blocks to 100% of peers.