* Added new config attributes: 'SecondsPerBlock','LowPriorityThreshold'
* Added new files:
* Added new method: CompareTo
* Fixed empty Slice case
* Added new methods: LessThan, GreaterThan, Equal, CompareTo
* Added new method: InputIntersection
* Added MaxTransactionSize, GroupOutputByAssetID
* Added ned method: ScriptHash
* Added new method: IsDoubleSpend
* Refactor blockchainer, Added Feer interface, Verify and GetMemPool method
* 1) Added MemPool
2) Added new methods to satisfy the blockchainer interface: IsLowPriority, Verify, GetMemPool
* Added new methods: RelayTxn, RelayDirectly
* Fixed tests
* Implemented RPC server method sendrawtransaction
* Refactor getrawtransaction, sendrawtransaction in separate methods
* Moved 'secondsPerBlock' to config file
* Implemented Kim suggestions:
1) Fixed data race issues
2) refactor Verify method
3) Get rid of unused InputIntersection method due to refactoring Verify method
4) Fixed bug in https://github.com/CityOfZion/neo-go/pull/174#discussion_r264108135
5) minor simplications of the code
* Fixed minor issues related to
1) space
2) getter methods do not need pointer on the receiver
3) error message
4) refactoring CompareTo method in uint256.go
* Fixed small issues
* Use sync.RWMutex instead of sync.Mutex
* Refined (R)Lock/(R)Unlock
* return error instead of bool in Verify methods
