Reducing cruft and sorting media files

Not perfect, but it's a start.

The goal would be to have all vector source files in media/yeoman-source, all files used in the site in assets/img, pre-optimized files for other sites / users to grab and use in media/yeoman-optimized, and vendor source files in media/vendor.

We're missing .ai/vector files of some of the Yeoman poses and secondary assets (bookshelf, blocks, etc.) — anyone know where those are?

I removed a couple files that were duplicates, partials of other files, or out of date and unreferenced. There are plenty more that can probably be culled, but that will work itself out during redesign I think.

I moved some assets used in the wiki to assets/img. This I wasn't sure of — both keeping the referenced files in media and moving to the jekyll site assets dir seemed a little dirty to me. The files are:

Getting-Started.md
`12:  <img class="full" src="https://raw.github.com/yeoman/yeoman.io/gh-pages/media/workflow.jpg">`

Homepage.md
`4:<img src="https://raw.github.com/yeoman/yeoman.io/gh-pages/media/yeoman-horizontal.gif" alt="Ye
oman">`

docs/Generators.md
`27:<img src="https://raw.github.com/yeoman/yeoman.io/gh-pages/media/yobox.png" style="width: 100%; max-width: 400px;">`

If moved, the references will have to be updated there.