Refactor client, move each API to its own .cpp

This was way, way past due.
Add ID Verifier API

No implementation yet; just create the API
Refactor client, move each API to its own .cpp

This was way, way past due.
Merge branch 'develop' into vote

Conflicts:
	libraries/client/blockchain_api.cpp
	libraries/client/client.cpp
	libraries/client/debug_api.cpp
	libraries/client/general_api.cpp
	libraries/client/include/bts/client/client_impl.hpp
	libraries/client/mail_api.cpp
	libraries/client/network_api.cpp