Pass width to the embed script

Summary:
We were only using width for the iframe style but not telling
the program about it. This tells the program about it.

Test Plan:
- Create a program that has println(width)
- Embed that program with Perseus widget with custom width
- See the println() show the correct width

Reviewers: aria

Reviewed By: aria

Subscribers: #perseus, #cs

Differential Revision: http://phabricator.khanacademy.org/D13844