Add lib/screenshot.js

Based on:
http://hg.mozilla.org/qa/mozmill-tests/file/905bc0ba7a1e/firefox/lib/screenshot.js
lib/screenshot.js: remove use of utils module
lib/screenshot.js: use MOZMILL_SCREENSHOTS environment variable

Use the MOZMILL_SCREENSHOTS environment variable to select the directory
where to store screenshots.
tbbScreenshot.js: use the new screenshots lib
Save mozmill json results and screenshots in reports directory

Add a --reports-dir option to define a reports directory.
README: add usage infos and options list
Create thumbnails of screenshots
Use common hash for mozmill and selenium tests
Keep bundle sha256sum in tbbinfos
Add general bundle test status
Add html tests report