Made SystemBitManager instanced

Changed SystemBitManager usage from static to instanced:
- solves a multithreading issue when several EntityWorlds are
concurrently initialized;
- solves an issue when every next EntityWorld created gets its System's
SystemBit starting not from 0, but from the last bit of the System of
the previous world (i. e. SystemBit value gradually increases between
worlds).
Merge pull request #59 from Maximusya/master

Made SystemBitManager instanced