Simple mode to shorten URLs from Emacs.

Adapted from bitly.el from Jorgen Schaefer <forcer@forcix.cx>
available here https://github.com/jorgenschaefer/bitly-el

Use (isgd-shorten URL) from an Emacs Lisp program, or
M-x isgd-url-at-point to replace the URL at point (or the region)
with a shortened version.