Cannot create Angle objects with decimal.Decimal

Angle constructors should be able to take `decimal.Decimal` objects.


    from astropy.coordinates import Angle
    import astropy.units as u
    from decimal import Decimal

    d = Decimal(12.34)

    a = Angle(d, unit=u.degree)
    print a.hms

This fails with:

    ERROR: ValueError: Value not scalar compatible or convertable into a float or integer array [astropy.units.core]

It is, of course, pretty easily convertible into a float. Also, the word “convertable” is misspelled in the error above.