Add gitignore
Add Negotiation dep
Add ContentNegotiation middleware

New environment variables:

* 'request.best_format': the best format you can use to respond to the client;
* 'negotiation.priorities': configuration variable for the Negotiation middleware.
Move ContentTypes middleware to a better folder
in order to be autoloaded
Merge pull request #1 from willdurand/content-negotiation

[RFC] Content negotiation middleware + minor tweaks