Supports decoding RGB, CMYK and YCCK images.

[harukasan]

Go 1.5 adds to support decoding RGB, CMYK and YCCK JPEGs.

• Supporting RGB adds here: https://github.com/golang/go/commit/cc009687bc30793097166b4aaf21ed34d7192809
• Supporting CMYK and YCCK adds here: https://github.com/golang/go/commit/f10e03770c9702b3ee3bcc2ccdb78691f389c1a3

We should follow this change, but image.CMYK struct is provided only Go 1.5 (added here: https://github.com/golang/go/commit/b5c3a9e572a1257c0db47d74b45f8e03f2f91f27) and later, probably.

We can not bind library version to runtime version in current Go build system, so we have some plans:

• Implements own image.CMYK struct that implements to image.Image interface.
• Support only Go 1.5 and later.
• ...

[edvakf]

Support only Go 1.5 and later.

I prefer this. There is no reason people can't upgrade go version.

[harukasan]

RGB JPEG is now supported at #14.