Cleanup DefaultAssetPath

Add automatic locking / unlocking of accounts

Validate block header gas limit

* Add block header gas limit validation in ValidateBlock function,
  see eq 39 and 45 in yellow paper. Before it was calculated _for_
  the block instead of validated.
* Use the block header gas limit when setting the gas pool instead
  of calculating the value for the block.

* Use RWMutex instead of Mutex
* Use time.Duration instead of int for unlock time
* Use time.After with select instead of time.Sleep

* Change account signing API to two sign functions;
  Sign without passphrase - works if account is unlocked
  Sign with passphrase - always works and unlocks the account
* Account stays unlocked for X ms and is then automatically locked

Update Go and Qt versions in Travis

RPC cleanup

Fix key store address hex decoding and accounts test

Thanks to https://github.com/jaekwon for original fix!

Convert gas values to integers

Implement OS sensitive dataDirs

Allow zero and negative block numbers

topics fix

Fixes #370

* Added an additional tx state which is used to get the current nonce
* Refresh transient state each time a new canonical block is found
* Properly uninstall filters. Fixes a possible crash in RPC

0 is genesis block. Xeth recognises -1 as current

Correct stack size validation for MUL, CALLDATACOPY, EXTCODESIZE, BLOCKH...

Persistent Node Key