Implement sign-in screen for WMT client

To make it clear to users that they need to be signed in to use WMT, I've added a sign-in screen. It's the first thing a user sees when accessing WMT. Once signed in, the WMT client GUI is shown. On sign out, the sign-in screen is shown again. Previously, if a user wasn't signed in, they could still see the GUI, but it was unresponsive--an unattractive workaround.

![screen shot 2014-07-16 at 4 50 07 pm](https://cloud.githubusercontent.com/assets/5673711/3606789/b34a7f36-0d3b-11e4-8ed3-6b78a03a73e9.png)