xbuild: use the AdditionalReferencePath items to locate assemblies

This is a little underdocumented feature of MSBuild. Items with the name
"AdditionalReferencePath" get searched for referenced assemblies. This helps
with some scenarios, e.g. when the assembly has to be references from a central
location.
Merge pull request #649 from DavidS/feature/implement-additional-reference-path

xbuild: use the AdditionalReferencePath items to locate assemblies