Place for API documentation.
Added API docs

This starts an outline of the API docs.

The formatting is Markdown as described by the latest Apiary Blueprint
format:  http://apiary.io/blueprint

This format allows for:
  * Easy to type/format
  * Machine readable/parsable docs
  * Viewable on Apiary.io

For local viewing, kept the ability to just look at it via /docs.  But
Apiary.io is much nicer and gives a UI for easy testing.
Merge pull request #57 from austinylin/master

Order conversations newest update first by default
Set a sensible default INCOMING_EMAIL_DOMAIN.
Merge branch 'master' into pr/56
Merge pull request #58 from drewblas/api_docs_basics

Added API docs
Setup MessageMailer to replace ConversationMailer