New editor & viewer

ACE editor website: http://ace.c9.io/
Use ACE - JavaScript based editor for writing new submissions, view snippets when viewing submissions

## Changes

1. Use ACE editor in place of default textarea box while writing new submissions
2. Use ACE editor to view code snippets in submissions instead of `Pygments` syntax highlighter
3. Remove `submission.models.Revision.item_highlighted_code` field
4. Remove `submission.models.DisplayFile`, `submission.views.get_display`, `submission.views.show_file`

## Notes

1. `Pygments` package is no longer used for syntax highlighting code snippets.
2. Redundant storage of snippet is removed in `Revision` model
3. Server load to syntax highlight is reduced. However, page load time is increased little bit to load ACE editor
4. `submission.views.show_file` and its related methods, models appears to be broken and not used anywhere. This is discussed in the below comments

## Preview

### Snippet view

![code1](https://cloud.githubusercontent.com/assets/932927/2658123/a5732194-c004-11e3-9076-0a4e1f0ec458.png)

### Submission editor

![code2](https://cloud.githubusercontent.com/assets/932927/2658124/a57756ce-c004-11e3-9a94-f43c2627b4ed.png)