Dashboard authentication

I'm opening a pull request just to keep track of progress for dashboard authentication; when it's merged it should fix #21.

* As a start, I've added a new configuration section (`Dashboard Parameters`) with configuration options `login_username` and `login_pw`. Both options default to empty strings. 

* Then, in `dashboard_server.py`, I added functions to handle authentication (mostly taken from Jay's tip about http://flask.pocoo.org/snippets/8/ ) and decorated all the routes to require authentication.

* I added in behavior to not require authentication if either the username or password is blank -- the idea being that if someone wants to turn on auth, they should provide both a username and password.

* I briefly tested this on Chrome and it appears to work, but it should probably be tested a little more rigorously and on other browsers. 

* Also, the dashboard itself needs an interface to modify the username/password. I don't know coffeescript, though, and I'm not sure where the best place to add it would be -- create a new sidebar tab for dashboard configuration? Add it to the existing dashboard tab? Add it under server configuration?