Add pillow 2.4.0 support

Pillow 2.4.0 added support for JPEG2000 using OpenJPEG 2.0. Because
Pillow calls the format JPEG2000 instead of JP2, we need to rename it to
enable the optimized code path. Should still be backwards compatible.
Merge pull request #12 from felixbuenemann/pillow-2.4.0-support

Add pillow 2.4.0 support