[WIP] Docs refactor

For https://github.com/pypa/python-packaging-user-guide/issues/5

Since virtualenv only consists of two/three commands at its heart (`virtualenv`, `activate` and `deactivate`) I thought a "quickstart" for these, without explaining what the heck they do and how to use them, would be somewhat unhelpful.

WIP - 

 - [ ] look over options and make sure they're ok
 - [ ] add all necessary env vars (needed?)
 - [ ] refactor the explained options from the bottom of the userguide to reference? (e.g --extra-search-dir, --relocate)
 - [ ] Write a proper module documentation for `virtualenv` (in relation to its extensibility)?
 - [ ] Other sections could be removed / need major rewrite?

Request for comment, on the above