Fixed maxlength in forms charfield

Looks like django 1.4 has some flows :( Hopefully this patch will address @mikewest issue.

TEST=Verify you can't enter more than 500 characters in `summary`, `doc_links` and `sample_links` fields in the /admin/features/new page.

BUG=#106

![image](https://cloud.githubusercontent.com/assets/634478/3809931/d58ff226-1c89-11e4-8f27-eb804dc42236.png)