Merge @tyrelsouza's restricted tag selection branch, minor readme fixing, make setup.py executable

This pull request includes three things:

* It merges @tyrelsouza's branch for "restricted" tag selection.  This allows you to have fields that only allow existing tags to be used, and not new tags, when you set `restricted=True` on the `TaggableManager`.  It is disabled by default.
* It fixes a couple of restructuredText-isms in the README.md file.
* It adds a hashbang and makes setup.py executable on *NIX systems.