Add support for clip (Windows)

See tremby#18

Document the new ability to upload via URL

Reformat readme

Merge pull request tremby#7 from tremby/no-require-display-on-mac

Don't require DISPLAY if pbcopy can be used

Return an HTTPS image URL rather than HTTP

Updates for the v3 API

Closes tremby#3

Version 5

As found on my workstation at the time of tagging, believed not to
have been edited since rolling in Tino Sino's suggested changes in
October 2012.

v4

As sent to Imgur on June 2012, with the message:

> Hi, Alan. Here's another new version of that little script -- I had an
> error report from a user that some versions of Bash don't like the
> usage function having a string split over many lines.

Version 2

As originally sent in December 2009 to Alan on his request for
user-made Imgur apps

v2+gist

As uploaded to Gist in January 2012