Add support to custom HTTP methods

Tornado CORS ignores custom HTTP methods, because currently supported methods are hard-coded.

This patch generalizes supported HTTP methods using the attribute SUPPORTED_METHODS, available at Tornado's RequestHandler class [1].

This "bug" was found while implementing the "PURGE" method at Brainiak API [2].

[1] http://www.tornadoweb.org/en/stable/web.html
[2] Soon available at http://github.com/globocom/brainiak