Add `make server-offline ...` to makefile so you can run perseus without an internet connection

Test Plan:
Disconnect from the internet
Run `make server-offline PORT=9000`
Visit http://localhost:9000 and be amazed

Reviewers: aria

Reviewed By: aria

Differential Revision: http://phabricator.khanacademy.org/D12737
Merge branch 'master' of https://github.com/Khan/perseus

Conflicts:
	Makefile