Stake module implementation

[shanev]

Fixes #6.

Initial implementation of the stake module. Introduces the basic types. ABCI functions will be handled in #9.

This is mostly so work on the indexer can start without finishing all stake functionality. So there's a a bunch of TODOs and commented code.

[jhernandezb]

I think we haven't discussed if we are going to add some custom events to be picked by the indexer, do you think should be handled on a different issue?. I could see some events like post created or delegation expiration.