Add image rotation using exif information

Ive added image rotation to diaporama using exif tags. 

Rotation can be switched off using the plugin configuration. 

Support for extended exif rotations that needs mirroring is added but only works for php version of at lease 5.5 (imageflip function is needed). For lower php versions only the exif rotations that needs rotating only are implemented.

Rotation of the preview image (not thumbnail) is done using css3 tranforms.