PhabricatorSlug

Summary:
This is to be used in Phame so the logic is shared where possible. The change has three main things going on

• broke out functionality from PhrictionDocument that isn't Phriction specific.
• swept up code base to use new PhabricatorSlug class.
• altered the regex ever so slightly per discussion and http://stackoverflow.com/questions/2028022/javascript-how-to-convert-unicode-string-to-ascii

I think maybe we should punt on unicode here for quite a bit -- http://www.456bereastreet.com/archive/201006/be_careful_with_non-ascii_characters_in_urls/ -- but we'll be well-positioned to add it with the code here.

Test Plan: used phriction to create, edit, view documents. used a tool (codemod) for the codebase sweeping

Reviewers: epriestley

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2195