[BrowserTest] Added support for uploading the language screenshots

To use this script:

*Create an environment variable MEDIAWIKI_API_UPLOAD_URL having API URL
of the site where the images are to be uploaded. (Eg: http://commons.wikimedia.beta.wmflabs.org/wiki/api.php)

*LANGUAGE_SCREENSHOT_CODE environment variable should be set.

Run the upload script by: bundle exec ruby upload.rb

Bug: 62737
Change-Id: I1937aa5f02e3bf587ac65d8d11c3ebbb8b1d0528
Merge "[BrowserTest] Added support for uploading the language screenshots"