Update ExtCore package to 0.8.35 to take advantage of recent improvements.
Choice.setError was renamed to Choice.error in newer versions of ExtCore.
Upgrade NuGet client and build targets to 2.7 (2.7.40906.75).
Small tweaks to paths so NuGet package restore works correctly from the command line when called from xbuild/Mono.
Merge branch 'master' into graham-optimized