Fixed issue #230 -- prevent crash when transaction is in error state

fix for ticket https://github.com/django-debug-toolbar/django-debug-toolbar/issues/230
if isolation_level is looked up while transaction is in an error state, an exception is thrown, which eats up the original exception that is never shown
Fix doc for show_toolbar.
fix hashcompat deprecation warnings with django 1.5
Merge pull request #291 from valsteen/master

prevent crash due to SQLDebugPanel when transaction is in error state
Merge pull request #336 from craigds/master

fix hashcompat deprecation warnings with django 1.5
Merge pull request #335 from fossilet/master

Fix doc for show_toolbar