Psmobile small fixes

Applied fixes described in http://community.monogame.net/t/spritebatch-cs-effect-cs-this-does-not-appear-to-be-a-monogame-mgfx-file/1070/2

Small fix to be able to use XNA XNBs without having to "manually edit them as stated in http://chainedchaos31.tumblr.com/post/97557910457/playing-audio-on-the-playstation-vita-via-monogame

Hi Tom,

I think that the problem is that the PSM shaders are not MGFX compliant, thus accessing to ReadHeader crashes badly. Also, there's no access to header.HeaderSize.

Would it be better setting variables first and then using the same code for both for the effect caching like this?   http://pastebin.com/85LRW03w