* 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
* Fixed small incosistencies related to comments and variable name
* For testing purposes we have to move the method newBlockchain from the cli/server/server.go to pkg/core/blockchain.go. In addition we have to rename it to NewBlockchainLevelDB in order to be able to export it and avoid naming conflicts. In future we still need to think how to switch between different blockchain implementation easily but for the time being this is not possible.
* Added unit tests for the rpc server
* Added unit_testnet chain fixture
* fixed port number
* Added errors handling
* move unit_testnet chain from 'cli/chains' to 'pkg/rpc/chains'
