Add additional matrix split to nuspec

Update targets file to only import if matching $Platform
When building cef.redist nuget packages update cef.redist.targets to include a unique target name for 32/64bit otherwise they don't seem to coexist happily
Remove item group entries, don't believe it's relevant anymore
Update cef.redist.targets file for 32bit and 64bit targeting (seems that target names need to be unique or nothing happens, at least in VS2013)