* Raise exceptions when conflicting arguments are used
* Build the auth headers when instanciating Gitlab, not on each request
* Enable anonymous Gitlab objects (#364)

Add docs and unit tests

Make gitlab objects serializable

With current implementation of API v3 and v4 support, some instances
have properties of type module and are not serializable. Handle
these properties manually with setstate and getstate methods.

Expose the X-* pagination attributes returned by the Gitlab server when
requesting lists.

Closes #304

These methods change the auth information and URL, and might have some
unwanted side effects.

Users should create a new Gitlab instance to change the URL and
authentication information.