Changes to IForceClient for new method and making it public.
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".

MediaFriendsInc2/Force.com-Toolkit-for-NET,wadewegner/Force.com-Toolkit-for-NET,NowBI/Force.com-Toolkit-for-NET,dcarroll/Force.com-Toolkit-for-NET,joshbooker/Force.com-Toolkit-for-NET,tugberkugurlu/Force.com-Toolkit-for-NET,nmonasterio/Force.com-Toolkit-for-NET,developerforce/Force.com-Toolkit-for-NET,jshepler/Force.com-Toolkit-for-NET,Compassion/Force.com-Toolkit-for-NET,madmax983/ForceToolkitForNet,developerforce/Force.com-Toolkit-for-NET,tugberkugurlu/Force.com-Toolkit-for-NET,NowBI/Force.com-Toolkit-for-NET,aaronhoffman/Force.com-Toolkit-for-NET,shankararunachalam/Force.com-Toolkit-for-NET,jshepler/Force.com-Toolkit-for-NET,caltomare/Force.com-Toolkit-for-NET,madmax983/ForceToolkitForNet,joeferraro/Force.com-Toolkit-for-NET