Add framework for different Notary Types

[von]

To add support for Convergence (Issue #44) and Google Certificate Catalog (Issue #1) a framework to handle different notaries is needed.

I think this framework should have a abstract Notary interface with implementations for Perspectives, Convergence and GCS. The interface would allow asynchronous query.

Then the framework is configured with a set of notaries, a point value for each, the point value required and a timeout. It would query all notaries, for each that succeeds it would add the points associated with it to a total. If the total reaches the required, success (addressing Issue #33). If timeout is reached, or all notary queries complete without reaching the minimum, failure.