Add JPEG 2000 support

JPEG 2000 support using OpenJPEG library.

- Add read/write support to JP2/J2K format for Gray/RGB(A)/CMYK images @8/16-bits
- Read test case images generated by Photoshop

The plugin has the following limitations:
- Resolution is not set (JP2_RES box is marked "For the future" in jp2.h)
- Metadata are not set (as with resolution)

Closes #13

src/imageformats/jp2.json

@@ -0,0 +1,4 @@
+{
+    "Keys": [ "jp2", "j2k", "jpf" ],
+    "MimeTypes": [ "image/jp2" ]
+}