Implemented an authenticity_param option on AuthenticityToken

Hi,

I've implemented an `:authenticity_param` option on the `AuthenticityToken` protection to allow for a different param name to be sent/expected by the application that uses it.

It provides some flexibility for users that would like to use a different naming convention on their form data.

The same could be applied for the header `X-CSRF-TOKEN` if you consider it necessary.

Hope you find it useful,
Darío