Allow time format and scale strings to be case-insensitive

I bumped into this while trying to demo something in the time module, and was surprised to find `Time(... format='JD')` did not work.  It expects `'jd'`.  Given that this format (and many of the others, as well as the time scales) are acronyms it seems natural to pass them in as all-caps, instinctively.

It also makes sense to me internally to just use lower-case names consistently but the constructors at least should accept all caps I feel.