New types Account, Currency, Directory:
* New tagged uint types.
* Extract to_string functions from header to hide dependencies.
* Include what you use and C++11 for cleanups.
Update state during pathfinding
During playback close ledger before accepting
Restore checkpointed ledger under all paths in PathFind.
Tighten up some serialization checks
Improvements to HTTP parsing
Structured Overlay support:
* Add Peer Protocol detector
* Add RIPPLE_SINGLE_IO_SERVICE_THREAD setting
* Preliminary HTTP header parsing logic (disabled)