Add build files

I created the following to enable automated builds and to enable you to open the projects on "initial" checkout.

build.proj:
This is the msbuild file which does all the magic. 
 - It will download a nuget.exe file if it can't find it 
 - restore the xamarin.forms packages where the targets files are referenced
 - build the solution in release mode
 - create nuget packages of the output

build.bat:
To be used on windows. Calls the build file with msbuild 4.0

build.sh
To be used on linux (and mac I suppose). Calls the build file with xbuild

I haven't tested the build.sh file yet as my Linux boxes are busy and I need to spin up a new one.