replaced nginx with tornado in docker image.

- resulted in smaller image size (tornado was already installed for ijulia)
- implemented file upload/download and directory browsing with tornado
wait for shell to start before ponding to browser.
also removed (hid) the option to re-create a session from the login screen to simplify the screen.
synchronize git repos. admin enhancements.

- added sync tab with ability to synchronize git repositories on to juliabox
- enhanced admin/account tab to display ssh public key (for git authentication), juliabox version.
- allow user to upgrade juliabox image
Merge pull request #19 from tanmaykm/tanmaykm

git sync and other enhancements