Refactor simplify `define_token_authentication_helpers_for`

Remove the need for the `get_entity` helper method, make
easier to use variables in the methods definitions.
Minor refactor move fallback default to configuration

The default value of the `fallback_to_devise` remains static, but
it is now stored among the other configuration options.
Refactor use `:fallback` option instead of `:fallback_to_devise`

The change is internal and does not affect how the gem behaves.

Strictly speaking, the `:fallback` option can now be used from
`handle_token_authentication_for` in place of `:fallback_to_devise`;
however, since it is still undocumented, there is no reason to
notice the extension and the gem behaviour can be considered unchanged.
Refactor inject fallback auth. handler in helpers definition

Remove the responsibility of defining the fallback authentication
handler from the `define_token_authentication_helpers_for` method.