ENH: auto expand ace-editor height

1. As the code length increases, the height of editor expands automatically.
ENH: Ajax reST preview for new submissions

1. add `scipy_central.rest_comments` module to `INSTALLED_APPS`
2. rewrite `rest_comments.views.rest_to_html_ajax` view
3. `/static/rest_comments/rest-actions.js` a generic reST interface to convert content. `scipy_central.comments` module can be ported to it.
4. Ajax previewer for `description` field for new submissions
Merge pull request #171 from ksurya/ajax-preview

Ajax preview Form Submission