Templates and Protobuild Fixes

This PR has the latest Protobuild.exe fixing store certification for WP8.1 projects and support for ConfigurationMapping entries.

It also makes the `<MonoGameContentBuilderExe>` path overloadable from a game's csproj file.

Finally I fixed the manifest for the WP8.1 template to correctly use template parameters.

Fixes #3237.
Fixes #3146.