[webcomponents]: Minutae -- moved proposals from wiki to /proposals

Folks,

To alleviate the limitations of the github wiki (can't submit PRs easily,
etc.), we are moving to a slightly different, more streamlined (awesomer!)
model: the /proposal directory:

https://github.com/w3c/webcomponents/blob/gh-pages/proposals/

It's basically the same as the wiki, but now you can contribute with the
traditional fork/pull model.

:DG<