Add jansky module to convert AB mag to mJy

Two function to either convert AB mags to µJy or convert pixel
values (DN) toµJy.

These functions are imported globally by the package.
Add ignore file
Add WIRCAM_VEGA_TO_AB dictionary

This dictionary encodes ZP offsets, from Vega mag to AB mag for
2MASS/WIRCam filters as computed by Blanton et al 2005 AJ 129 2565.

This dictionary is imported in the top level of androphotsys.
Make a wircam_vega_to_ab function wrapper available