Add support for multiple Envato accounts

Merge pull request #1 from Prelc/master

Tested on OS X Yosemite.

Update README.md

Added that the first argument is the purchase code. If missing, the script prompts of the input

Update README.md

Updated screenshot

Added the user agent header to the curl request. Suggested by dtbaker TF author.

Added screenshot to README

Added screenshot.

Updated README

First commit.

Initial commit