Migrate/Implement Enigma transaction support

[r3rcloak]

Porting/implementing support for Enigma transactions represents a significant piece of work in relation to the rebasing of Cloak to the newer Bitcoin codebase. As this work will likely involve considerable code reworking and revision, it may be wise to look to decouple the Enigma related code during the porting process to decouple the Enigma related functionality from the underlying base code as much as possible.

Enigma.cpp/Enigma.h contains the top level code related to Enigma sends (see the SendEnigma method) in the aforementioned files for the start/entry point for sending an Enigma transaction.