Add ActsAsTokenAuthentiactionHandler specs
Refactor specs: DRY by extracting helper methods

A `if Rails.env.test?` condition is used to allow Cucumber to check
for the Devise `sign_in` method. That's much of a dirty hack, lets
ignore it for now, hoping the new unit tests suite will make those
scenarios redundant.
Refactor specs: DRY by extracting more helper methods
Refactor specs: DRY by extracting even more helper methods

Gather most of the metaprogramming effort in a few helper methods
Refactor move spec setup from let() to before(:each)

The `let`-based setup was sensible to the order of creation of
the variables. The correct order was ensures by the occasional
use of `let!`. That important factor was almost invisible.

The `before` blocks are easier to read and their behaviour
is also easier to understand IMHO.
Minor refactor remove redundant setup statements

Since the `before` blocks are always executed (the `let` block
were not), there is no need to repeat the stubbing which was performed
as part of the context setup.
Refactor specs: add dummy classes helpers to support

Apply the move from `let` to `before` to the SimpleTokenAuthenticatable
module specs.
Add DummyTokenGenerator specs

Ensure it behaves as a token generator.
Add usage documentation to dummy classes specs helpers
Merge branch 'add-acts-as-token-authentication-handler-specs'