Add YouTube embedding

Hooks into the existing image embed system with its show/hide.

Embed using the Markup image embed syntax:
Eg. ![](http://www.youtube.com/watch?v=NFZY_o0QoJk)

Supports only youtube links of the form:
youtube.com/watch?v=[embed_id]
Merge pull request #93 from stevetan/master

Add YouTube embedding
append @username when you click on user
Merge branch 'master' of github.com:ytamura/prat
deleted comment line
Merge pull request #94 from ytamura/master

Appending @username by clicking on name
refactor user prefs into backbone views
Preview of embedded video works properly

This fixes the preview you get when you click the preview button with an
embedded YouTube video.
Merge pull request #95 from stevetan/fix_video_preview

Preview of embedded video works properly
Add pratlog backup script to text file

Usage: python -m chat.scripts.backup_pratlog -d '2013-08-24'

Backup the given date of chats into a txt file named:
  pratlog.[channel name].[date].txt
Log script:Add docstring, use .format() for string
Log script: Move Mongo initialization into method
Log Script: Add alternate config support
Log Script: Add log dir option, fix file handles

setdefault's second argument is always called, causing the file to be open
multiple times
Log Script: Better docstrings for script and args
Merge pull request #96 from stevetan/pratlog

Pratlog
Merge remote-tracking branch 'upstream/master'

Conflicts:
	chat/assets/preferences.coffee
	chat/templates/index.pre.htmljinja