Add contribution guide. [ci skip]
Document #reply_privately in README. [ci skip]
Bump version to 2.4.0. [ci skip]
Add new API for Source objects that allows explicitly marking them as private. Fixes #18.
Use new Source API internally.
Add Source#private_message!
Use new Source API for Message.reply_privately.
Merge branch 'master' of https://github.com/jimmycuadra/lita
merging upstream