Correct gas limit validation according to new algorithm

* Use absolute value of (block's gas limit) - (parent's gas limit)
  in comparison with diff limit.
* Ensure the diff is strictly smaller than the allowed size.

Conflicts:
	rpc/api.go

RPC Args

* CALLDATA use getData
* removed old context get range value
* removed casting big => int for some cases
* pc now big int #457

Minor fix

Update README.md

Update README.md

settable etherbase

Use ExtraDB for RPC storage. Fixes #577

Update response types + tests