OAuth1.0 RequestValidator multiple docs fixes

Here are multiple fixes to issues that I found during implementation of OAuth 1.0. RequestValidator in my project.

* added missing methods in list of methods to implement (RequesrValidator class docstring)
* updated docstrings of some methods. This relates mostly to issues that were "documented" only on issue tracker (#185, #202) and should be explained in docs.
* added example implementation of `validate_realms`