Move referenced assets from media to assets/img

Assets for files used in the site, media for source and supporting files. NOTE:
the wiki will need to be updated to point to assets for:

- media/workflow.jpg -> assets/img/workflow.jpg
- media/yeoman-horizontal.gif -> assets/img/yeoman-horizontal.gif
- media/yobox.png -> assets/img/yobox.png
Remove duplicate images

- assets/img/mascot.jpg, duplicate of yeoman-003.png
- assets/img/yeoman-006.png, duplicate of yeoman-004.png
- media/yeoman.ai, contained in yeoman-logo.ai
- media/yeoman.pdf, duplicate of yeoman-logo.ai
- media/yo-grunt-bower/* minor unused variation of higher res vendor files
- media/yo-logo.png, not useful png of full yo-logo.ai
Remove unreferenced cli screenshots

Neither are of the current cli anyways.
Sort media files into vendor, yeoman-source, and yeoman-optimized

Not perfect but it's a start.

- Files that should be used to generate other files are in yeoman-source
- Files generated from yeoman-source are in yeoman-optimized
- Vendor source files are in vendor