added WPA81 to the list of PCL targets
updated nuget packaging instructions to include wpa81
put MinimumVisualStudioVersion back to 11.0
Merge pull request #575 from hippiehunter/master

added WPA81 to the list of PCL targets
Class PullRequest, renamed property Mergable to Mergeable (misspelled)
Merge pull request #576 from jrowies/master

Fix for mergeable property returning always false when getting pull request details
correct the reference to the NSubstitute package
another lost reference to NSubstitute