Enable optional build of NuGet packages without Xamarin components
Enable optional build of NuGet packages without Xamarin components

Allows for local source builds of B* NuGet packages without the need for
Xamarin to be installed / licensed.

NOTE that the package metadata still specifies support for iOS/Android,
but the libraries themselves will not be present in the Patlform nuget
package.

Closes #250

BrightstarDB/BrightstarDB,BrightstarDB/BrightstarDB,BrightstarDB/BrightstarDB,BrightstarDB/BrightstarDB