Extract Common Credential Module

[mdaines-r7]

There is some shared functionality between the Credential and Shared Credential class. Perhaps this can be split out into a module. In particular, the gem currently only exposes the test method for shared credentials.

I would call the common module (or class, if that makes sense) Credential, and then rename the existing class with that name to SiteCredential.

Then, in order to have both use the test method, make sure that both implement a to_hash method which provides the required format for testing a credential.

[mdaines-r7]

Pushed in 0.9 release.