Move test into src/test/java
Fix more XSD declarations
Add RequestEnhancer as customization point for token and auth requests

Example usage...

First configure the *TokenProvider you need with a RequestEnhancer (up to two for auth code):

    DefaultRequestEnhancer enhancer = new DefaultRequestEnhancer();
    enhancer.setParameterIncludes(Arrays.asList("foo"));
    tokenProvider.setAuthorizationRequestEnhancer(enhancer);
    oauth2RestTemplate.setTokenProvider(tokenProvider);

(only the auth code provider has the authorizationRequestEnhancer)

Then for each HTTP request that might require an access token to be obtained, set the extra
parameters on the token request

    oauth2RestTemplate.getOAuth2ClientContext().getAccessTokenRequest().set("foo", "bar");

And finally use the RestTemplate as normal

Fixes gh-123
Add OAuth2RequestAuthenticator strategy for authenticating client HTTP requests

The default just adds an Authorization: BEARER ... header. Can be injected into
an OAuth2RestTemplate (in Java or XML config) via a setter, so no need for
additional namespace support.

Completes cientside part of gh-136
Add TokenExtractor strategy to OAuth2AuthenticationProcessingFilter

Completes the server-side component and fixes gh-136
Fix a couple of integration tests