Changes to IForceClient for new method and making it public.

Having the IForceClient interface public makes it easier to mock the ForceClient when testing.
Adding in the new method that was added "UpsertExternalAsync".
Update Package-DeveloperForce.Chatter.nuspec
Update Package-DeveloperForce.Common.nuspec
Merge pull request #10 from JasonRowe/IForceClient_Changes

Changes to IForceClient for new method and making it public.