added support for explain.

Conflicts:
	src/MongoDB.Driver.Core/Core/Operations/FindOperation.cs
count explain doesn't work until server 2.7.6, so requiring that as a minimum version for the test.
added support for translating auth from connection strings and MongoCredential into IAuthenticators.
Merge branch 'auth'
added GSSAPI into core and refactored all core passwords to use SecureString.
CR Changes
CR Changes