Added NuGet reference to 'MSBuild.Microsoft.VisualStudio.Web.targets' to....

... eliminate the requirement on VisualStudio being installed.

#19 Build script fails if VS2012 isn't installed.
- Added NuGet reference to 'MSBuild.Microsoft.VisualStudio.Web.targets'
(http://www.nuget.org/packages/MSBuild.Microsoft.VisualStudio.Web.targets/)
to eliminate the requirement on VisualStudio being installed.