New editor & viewer (DONT MERGE)

ACE editor website: http://ace.c9.io/
Use ACE - JavaScript based editor for writing new submissions, show code when viewing submissions

## Changes

1. Use editor when writing new submissions. 
2. Use editor for viewing code in submissions [NOT YET COMMITTED] 

## Notes

1. `Pygments` package is no longer required as the job is done in front-end
2. `scipy_central.submission.models.Revision.item_highlighted_code` is no longer required

### Advantages

#### Change 1

Ironically the present editor (simple text box) is never a good place to start writing or making changes. Now the new editor probably can be a place.

#### Change 2

1. Redundant storage of code snippet is removed
2. Server load on syntax highlighting is reduced. **However, page load time is increased little bit to load ACE Editor**

## 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)