Update README to be concise, consistent & accurate

:fork_and_knife:

In an effort to create a more useable README, the following has been adjusted:

- [x] use consistent markdown header syntax of #, ##, ###, ####, etc.
- [x] make section headers semantically correct
- [x] adjust language to be more consistent and readable
- [x] wrap most content at 80 columns for readability and consistency
- [x] move some content, like the roadmap, to the GitHub wiki
- [x] add link to the mailing list in the README
- [x] remove the "to document" section
- [x] add link + info about the supermarket cookbook
- [x] add info about redis 2.4+ dependency for sidekiq
- [x] document background jobs
- [x] document authentication
- [x] update content to be accurate and up to date with the project

Some content has been moved to the wiki:

-  [Background Jobs](https://github.com/opscode/supermarket/wiki/Background-Jobs)
-  [Deployment](https://github.com/opscode/supermarket/wiki/Deployment)
- [Chef Authentication](https://github.com/opscode/supermarket/wiki/Chef-Authentication)
- [Info about Accounts + adding additional providers](https://github.com/opscode/supermarket/wiki/Account-Linking-with-OmniAuth)
- [Roadmap](https://github.com/opscode/supermarket/wiki/Roadmap)
- [Changing the Vagrant VM Defaults  ](https://github.com/opscode/supermarket/wiki/Changing-the-Vagrant-VM-Defaults)