Change dependency system to browserify

Summary:
Changes perseus' compilation and dependencies to use browserify.

NOTE: This does not get rid of the many globals (window.Perseus
and properties) floating through perseus, but is a first step in
that direction.

To complete this, the local server was changed from SimpleHTTPServer to
beefy, which servers browserified javascript, and jsx/concat was changed
to browserify -t reactify.

While this complicates the process of running perseus from the command
line slightly, this complication also offers improvements in that you no
longer have to compile to view the source in chrome, thanks to
browserify's source maps (so there is much less reason to change to
precompiled code in test.html).

This also doesn't make perseus use the exact same require system as
webapp, but should at least allow for compatable requirements.

Test Plan:
 * run `make server` and visit test.html and testrender.html
 * run `make all` and visit perseus in devappserver
 * uncomment pre-built and pre-built with source maps and load test.html
 * try adding all the widgets in test.html

Reviewers: alex, alpert

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D5721