Vumi documentation needs to be updated.

**Not ready for review yet**

A lot has happened and our docs are trailing incredibly far behind. We need to bring these up to speed. 

Stuff that needs to be documented:

1. Application base class is completely undocumented.
2. Dispatchers, old style dispatchers needs be documented as being deprecated. The new endpoint based dispatchers should be documented, including how to write your own. The whole notion of `routers` for dispatchers has gone away with these dispatchers as well and that needs to be documented.
3. the vumi.config stuff 
4. heartbeat stuff that's been added to blinkenlights
5. vumi.persist (Riak & Redis)
6. the sandbox, more specifically the JS sandbox
7. the transports can be documented better.
8. A generic high level overview of Vumi, the different moving parts and their responsibilities.