Fix bug 1063213 - only flush rewrite rules on theme activation

* Don't flush rules on init (too costly)
* Don't do a hard flush (too destructive)

See http://codex.wordpress.org/Rewrite_API/flush_rules
Merge pull request #40 from craigcook/bug-1063213-flush-rewrites

Fix bug 1063213 - only flush rewrite rules on theme activation