Fix angle/unit parsing

This is a fix for a bug mentioned in #1387.

```
Angle('+6h7m8s', unit=u.degree)
<Angle [u'90d00m00.00000s' u'105d00m00.00000s' u'120d00m00.00000s']>
```

I don't know if it fixes the bulk of #1387, but it needs to be fixed regardless.