Partially implement OAuth 2.0 Service and Token

Write Service header, implement constructors, <code>getAuthorizeUrl</code>, <code>getAccessToken</code>, <code>signRequest</code>. Create stubs for <code>getAccessTokenClient*</code>

Add tests for <code>getAuthorizeUrl</code>, <code>signRequest</code>.

Implement <code>Token</code> class.