Add __unicode__ to Unit and Angle so that unicode(unit/angle) works in python2

Inspired by formatting-related PRs such as #1322 and #1434, a trivial further one, to implement `__unicode__`, which get called when someone does `unicode(something)`, for `Unit` and `Angle`. With this 
```
print unicode(u.m/u.s)
 m
 ─
 s
```
and
```
print unicode(Angle(['1h2m3s','4h5m6s'])), unicode(Angle('1d2m3s'))
[1ʰ02ᵐ03.00000ˢ 4ʰ05ᵐ06.00000ˢ] 1°02′03.00000″
```
Here, `unicode(a)` is slightly different from `a.to_string(format='unicode')` in that, like `str(a)` it returns one string containing everything rather than an array of (unicode) strings.