TimeCode and KeyCode support for EXR Metadata

Hi, This isn't a 100% finished product, but I just wanted to check that I was on the right track.

I've added some support for more EXR metadata attributes, and matched them to the existing TypeDesc where possible. I've also added in two new vecsemantics, for TIMECODE and KEYCODE, And assuming you agree with what I've done here, I'll also ensure that the DPX reader / writer behave in the same way.

Cheers,
Mark