* OS/X: Add script to run external diff; add space around tab widget; textual fixes * Bail out of easyhg.py in uisetup rather than at extension load time if we can't load PyQt4 -- that way we find out about it in easyhg * Ensure editing the default path works even if hgrc doesn't yet exist; ensure path and branch are reset (to empty and default) if their respective files are absent