Add relax= keyword argument to angle constructors

If set to `True`, then out-of-bounds angles (such as seconds >= 60,
as in `00:20:60.0`) result in warnings instead of exceptions. Many data
sets, including the Fermi GBM GRB catalog, give sexagesimal angles that
are improperly rounded.

The default is `relax=False`; if `relax=` is not specified, then
out-of-bounds angles continue to produce exceptions.

Example with default behavior:

    >>> ICRSCoordinates('00:60 00:60', unit=('deg', 'deg'))
    ERROR: IllegalMinuteError [astropy.coordinates.angle_utilities]
    Traceback (most recent call last):
    etc.

Example with 'relaxed' behavior:

    >>> ICRSCoordinates('00:60 00:60', unit=('deg', 'deg'), relax=True)
    WARNING:  [astropy.coordinates.angle_utilities]
    <ICRSCoordinates RA=1.00000 deg, Dec=1.00000 deg>