Issue #613

Frontier/sec 29

- add validation on TxMsg checking for nil
- add test for nil transaction
- add test for zero value transaction (no extra validation needed)

- rename Validate -> ValidateFields not to confure consensus block validation
- add nil transaction and nil uncle header validation
- remove bigint field checks: rlp already decodes *big.Int to big.NewInt(0)
- add test for nil header, nil transaction

- add validate method to types.Block
- validate after Decode -> error
- add tests for NewBlockMsg

Frontier/513

Configurable CORS domain

Actually verifies output as by regex

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

Improving this in the future will allow for cleaning up a bit of legacy
code.

Now formats based on underlying hexdata or hexnum type. Fields directly
with respective constructors that cover from native types