Add handling of Dwolla::RequestException

By wrapping and re-raising the `Dwolla::RequestException`
as a `CallbackException`, OmniAuth OAuth2 handles it well
(as an 'invalid credentials' failure) and allows the app's
authentication/authorization engine to process as needed.

For example, in the case of Devise, the default behavior is to
fail user authentication.  This can easily be adjusted based
on the wrapped exception to, for example, let the user know
that Dwolla payment permission was not received.

Without this change, the original `Dwolla::RequestException`
is passed up the chain.  For OAuth-based applications, the
exception is never handled in the OAuth layer.  In the case of Devise, this means the exception causes the application to
have an un-handled error, rather than refusing authentication as one would expect.
Added :provider_ignores_state=>true to options

Dwolla doesn't provide state info. As such, it must be
set to true for correct functionality with current omniauth.
bumped versions of everything

* updated omniauth-dwolla to 0.014
* updated omniauth dependency to ~> 1.1.4 (current version)
* updated omniauth-oauth2 dependence to ~> 1.1.1 (current version)
* updated dwolla dependency to >= 0.0.15 (current version)
updated tests

* added debugger as dev dependency
* added comments to dwolla.rb and dwolla_spec.rb
	regarding inability to set :provider_ignores_state by default.
* Rakefile now runs tests
* spec_helper now puts OmniAuth into test mode so the updated tests
	shared tests pass
Updated README

includes changes from
* Stefano Bernardi and Corey Innis
	( thanks guys )