JpgOutput support for controlling chroma-subsampling value

Ref: [[Oiio-dev] JPEG Output and chroma sub-sampling](http://lists.openimageio.org/pipermail/oiio-dev-openimageio.org/2014-October/013576.html)

Accepting an attribute called `jpeg:subsampling`, set to one of:
* 4:4:4
* 4:2:2
* 4:2:0
* 4:1:1

This allows the jpeg writer to explicitly set a chroma subsampling, while preserving the original defaults when not specified.  
I think it might be more clear to just say

    if (subsampling == "4:4:4")

just using regular string comparison. Strutil::iequals is specifically for case-insensitive comparisons (like, "OIIO" and "oiio" should be considered the same). There are no alphabetical characters in the strings you're comparing, so this is unimportant.