Added OpenRead and OpenWrite methods to IMvxFileStore. Implemented in MvxBaseFileStore, but there are more implmentations to be done.
Needs further review: Implementations of IMvxFileStore for WindowsPhone and WindowsStore - WindowsPhone implementation actually commented out.
Add null check in MvxBaseFileStore.WriteFileCommon.
Fixed calls to "this" in static methods. Changed 2 private static methods to instance methods.
Added exif rotation to the Android PictureChooser plugin

Added methods to properly rotate an image according to the exif rotation
data contained in the file when loading or taking a new picture.
Merge branch 'ExtendFilePlugin' of github.com:Blewzman/MvvmCross into v3.1
Modifications to unify delete behaviour for FileStream read/write - a bit scared of these changes...
Minor fixes to unify file experience - minimize risk of change (sorry!)
Minor fixes to unify file experience - minimize risk of change (sorry!)
Minor fixes to unify file experience - minimize risk of change (sorry!)
Minor fixes to unify file experience - minimize risk of change (sorry!)
Merge branch 'v3.1' of github.com:jsmarcus/MvvmCross into v3.1
Follow up to #627 - added a pokemon handler for now - it will drop back to the resampled/scaled picture if the exif code fails on some Android variant.