Add xpkg helper task

xpkg is a tool to create packages for the Xamarin.com component store
Merge pull request #107 from dnauck/master

Implemented the xpkg helper task
Fixing the xpkg task compile errors - relates to issue #107
Using the xpkg PackageFilename from the parameters - relates to issue #107
Merge branch 'master' into develop
Fixing bug in path resolution of the xpkg task - relates to issue #107
Merge branch 'master' into develop