Parse minutes or seconds equal to 60 with warning

Out-of-bounds angles with seconds == 60 or minutes == 60 result in
warnings instead of exceptions. Many data sets, including the Fermi GBM
GRB catalog, give sexagesimal angles that are improperly rounded.

Example:

    >>> ICRSCoordinates('00:60 00:60', unit=('deg', 'deg'))
    WARNING: An invalid value for 'minute' was found ('60'); must be in
    the range [0,60). [astropy.coordinates.angle_utilities]
    <ICRSCoordinates RA=1.00000 deg, Dec=1.00000 deg>

Some new test cases are included.

This addresses #947, without introducing any new arguments.
Merge pull request #990 from lpsinger/parse_seconds_60

Parse minutes or seconds equal to 60 with warning instead of exception