renamed addon to: "Rewrite HTTP Headers"

* the real addon is named: "moz-rewrite"
  (an obvious play on the name of the popular Apache httpd module)
* the git repo for this "lite" or "safe" version is: "moz-rewrite-amo"
  because this version is specifically for hosting on AMO
* the codebase actually refers to itself as: "moz-safe-rewrite"
  ..just because it seemed appropriate at the time
* however, this addon can't be named "moz-safe-rewrite" on AMO.
  I figured that "Rewrite HTTP Headers" would be a more descriptive name.
  also, more user friendly.
* this commit is simply a "rebranding"