Make hms/dms namedtuples and add `absdms`

This small change adjusts `Angle` so that the `hms` and `dms` attributes yield `namedtuple`s instead of regular tuples.  That enables usage along these lines:
```
'{0.h} {0.m} {0.s:.3f}'.format(coordinate.hms)
```

It also adds an `absdms` attribute that gives the absolute value of the `dms` elements.  This is valuable for string formatting purposes (this was prompted by an on-list discussion about how to make IAU-style formatted strings.)  Note that there is no `abshms` because I know of know convention where hour angle is negative.  But it could be easily added in.