added a new "Antenna Gain" equivalency (Jy to K)
fixed equivalency call
add to __all__
fixed tests & examples as per @mdboom's comment
fixed docstring tests: reduce # of significant digits
same as previous commit, but for the <Quantity object.  Unfortunately,
this hides the units...
added some more clarification & references to the jytok equiv
fixed doctest
name change: jytok -> brightness_temperature