* Added simple_backend.FSAccess class, which partially implements
  the SVNAccess API, but on a plain old filesystem. Needs tests
  and documentation, among other things (e.g. justification)

* Refactored SvnAccess to split out recently-added options for
  update_before/after_write into logically separate component

* SvnAccess.set_kind and .write now return the pysvn.Revision of
  the (last) commit instead of None

* Added SvnAccessEventEmitter class which executes a list of callback
  functions at the end of every successful .write and .set_kind action

  Callbacks should have the signature 
   (uri, contents, msg, kind, (pre_rev, post_rev))
