Implemented #csrf_meta_tags

- Implemented #csrf_meta_tags. It complies with [Rails implementation](https://github.com/padrino/padrino-framework/issues/1127#issuecomment-16068768)
and will generate two meta tags one with the `csrf-param` your app is
expecting and the other with the `csrf-token`. Fixes #1127 and #1251.
- Added a csrf_param setting to allow you to tell what your CSRF token's
  field name should be when sent as a form parameter. It defaults to
`authenticity_token`. `rack-protection` doesn't allow for that param to
be set, so if you do so and want the middleware to work correctly you'll
have to patch it or use your own validations for now - [see this](https://github.com/rkh/rack-protection/pull/65).
- Refactored and cleaned up the CSRF helpers' code base and some
  helpers' tests.