Add tests and interface

Also include stuff in 4.5 and mono csprojs
Add way to get to blob client
set Activity client accessor set to private

this isn't related to implementing the blobs client.  Just noticed it
doesn't match the rest of the properties.
Create ObservableBlobClient and interface
Write tests for observable blob client
Add blob client to ObservableGithubClient
Update monoandroid and monotouch csprojs