This also reduces the time required spend in the VM

The transaction pool will now some easily be able to pre determine the
validity of a transaction by checking the following:

* Account existst
* gas limit higher than the instrinsic gas
* enough funds to pay upfront costs
* nonce check

* Add params package with exported variables generated from
  github.com/ethereum/common/blob/master/params.json
* Use params package variables in applicable places
* Add check for minimum gas limit in validation of block's gas limit
* Remove common/params.json from go-ethereum to avoid
  outdated version of it

* Added new CreateAccount method which properly overwrites previous
  accounts (excluding balance)
* Fixed block tests (100% success)

* Refund of value

* Value XFER are refunded back to the sender if the execution fails

This will simplify the next commmit. Hash was only used for logging.

* All errors during state transition result in an invalid tx

* Refund should _always_ go to the origin

* PREVHASH => BLOCKHASH( N )
* State transition object uses VMEnv as it's query interface
* Updated vm.Enviroment has GetHash( n ) for BLOCKHASH instruction
* Added GetHash to xeth, core, utils & test environments