Consider adding GPS time scale

Coming out of [this discussion on astropy](http://mail.scipy.org/pipermail/astropy/2013-December/002944.html) there was a suggestion to add GPS as a time *scale* instead of just a time *format*.  As a time scale, GPS is 19 seconds behind TAI.  As a format it represents the number of TAI seconds since the GPS epoch.

Ideally we would stick with the three-letter acronyms for time scales so that the `gps` property would convert a time object to the GPS scale.  But `gps` is already used for the GPS format.  So either:

1. Find a different property name for the GPS scale, *or*
2. Rename the GPS format to something else, for instance `gps_sec` or `gps_seconds`

I like option 2 somewhat better, but of course it means an API change.  I think GPS is not the most widely-used format so maybe it's tolerable.

cc: @mhvk @astrofrog @eteq @scottransom