PR for issue #318 - Add PoW bundle (BLEEPlib)

The more information is written in #318 .

Add PoW consensus Protocol implementation within BLEEP library
Add integration tests, and unittests for PoW implementation
I have confirmed that PoW blocks are synchonized successfully with tests (regtest/3_BLEEPlib/3_PoW_treesync)
BlockTree data structure is the main PoW data structure shared among different nodes
BlockPool data structure is used for testing purpose

kaistshadow / blockchain-sim