Added PSMobile build tast.
Updated targets to clean down the obj directory before doing builds
Merge remote-tracking branch 'upstream/develop' into develop
Fixed the check for PSMobile
Corrected the PSMobile test (again)
Another fix for the default.build script
Last fix for the PSMobile test
Merge pull request #1706 from dellis1972/develop

Updated default build with PSM support
Merge branch 'develop' of https://github.com/mono/MonoGame into develop.helperclasses