Added a new "Antenna Gain" equivalency (Jy to K)

Jansky and Kelvin are often treated as equivalent units for a given telescope & observing frequency, and it is convenient to be able to convert between them.  No docs yet, just added a docstr with examples.

One problem I've already run into:
```
In [78]: u.K.to(u.Jy,jytok((50*u.arcsec)**2,5*u.GHz))
ERROR: UnitsException: 'K' (temperature) and 'Jy' (spectral flux density) are not convertible [astropy.units.core]
Traceback (most recent call last):
  File "<ipython-input-78-48f41af4e8f0>", line 1, in <module>
    u.K.to(u.Jy,jytok((50*u.arcsec)**2,5*u.GHz))
  File "/Users/adam/repos/astropy/astropy/units/core.py", line 523, in to
    return self.get_converter(other, equivalencies=equivalencies)(value)
  File "/Users/adam/repos/astropy/astropy/units/core.py", line 490, in get_converter
    self, other, equivalencies)
  File "/Users/adam/repos/astropy/astropy/units/core.py", line 454, in _apply_equivalences
    unit_str, other_str))
UnitsException: 'K' (temperature) and 'Jy' (spectral flux density) are not convertible

> /Users/adam/repos/astropy/astropy/units/core.py(454)_apply_equivalences()
    453             "{0} and {1} are not convertible".format(
--> 454                 unit_str, other_str))
    455

In [79]: (1*u.K).to(u.Jy,jytok((50*u.arcsec)**2,5*u.GHz))
Out[79]: <Quantity 0.0451337647146 Jy>
```