List of features.

[danibaena]

I think we should make a list of features for Piston, so users know what they can do with it. It will also be useful for the readme or docs. i.e.:

-Support for YAML/JSON Specs.
-Support form XML/JSON response.
-Combine different APIs specs.
-Possibility to use callbacks on the fly.
-Possibility to define custom extract fields from response on the fly.
-Capability to save the state (e.g. token for login actions).

Please add your own thoughts, I'm probably missing a few more features.