Make token generation pluggable

This should fix issue #6 . To change how tokens are generated you can
specify in your app.config a module that should handle the token
generation. The config looks like:

```erlang

    [
        {oauth2, [
            {token_generation, YOUR_TOKEN_GENERATOR}
        ]}
    ].

```

The eefault token generator is called `oauth2_token`. To implement your
own you should create your own module exporting one function
`generate/0`.

src/oauth2.erl

@@ -373,7 +373,7 @@ issue_token_and_refresh(Identity, ResOwner, Scope, TTL) ->
       Scope    :: scope(),
       TTL      :: non_neg_integer().
 issue_token(Identity, ResOwner, Scope, TTL) ->
-    AccessToken = oauth2_token:generate(),
+    AccessToken = oauth2_token_generation:generate(),
     ExpiryAbsolute = seconds_since_epoch(TTL),
     Context = build_context(Identity, ExpiryAbsolute, ResOwner, Scope),
     ok = oauth2_backend:associate_access_token(AccessToken, Context),

src/oauth2_config.erl

@@ -31,6 +31,7 @@
          expiry_time/0
          ,expiry_time/1
          ,backend/0
+         ,token_generation/0
         ]).
 
 %% Default time in seconds before an authentication token expires.
@@ -66,6 +67,12 @@ expiry_time(Flow) ->
 backend() ->
     get_required(backend).
 
+
+%% @doc Gets the backend for generating tokens.
+-spec token_generation() -> Module :: atom().
+token_generation() ->
+    get_optional(token_generation, oauth2_token).
+
 %%%===================================================================
 %%% Internal functions
 %%%===================================================================

src/oauth2_token_generation.erl

@@ -0,0 +1,41 @@
+%% ----------------------------------------------------------------------------
+%%
+%% oauth2: Erlang OAuth 2.0 implementation
+%%
+%% Copyright (c) 2012 KIVRA
+%%
+%% Permission is hereby granted, free of charge, to any person obtaining a
+%% copy of this software and associated documentation files (the "Software"),
+%% to deal in the Software without restriction, including without limitation
+%% the rights to use, copy, modify, merge, publish, distribute, sublicense,
+%% and/or sell copies of the Software, and to permit persons to whom the
+%% Software is furnished to do so, subject to the following conditions:
+%%
+%% The above copyright notice and this permission notice shall be included in
+%% all copies or substantial portions of the Software.
+%%
+%% THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+%% IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+%% FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+%% AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+%% LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+%% FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+%% DEALINGS IN THE SOFTWARE.
+%%
+%% ----------------------------------------------------------------------------
+
+-module(oauth2_token_generation).
+
+%%% API
+-export([generate/0]).
+
+-define(GENERATOR, (oauth2_config:token_generation())).
+
+%%%===================================================================
+%%% API functions
+%%%===================================================================
+
+%% @doc Generates a random OAuth2 token.
+-spec generate() -> Token :: oauth2:token().
+generate() ->
+    ?GENERATOR:generate().