Adding basic README.
Made everything work!

Now fetches the list of projects from the Codebase API and uses it to build the tree for tab completion.
Changing the icon for one that I made myself.  (In 5 minutes.)
Styling tweaks for the settings popup.
Making the large icon 128x128 so that the Chrome web store will accept it.
Another styling tweak.
Adding screenshot for publishing to Chrome web store.
Changing the way that the `username` input value is persisted in local storage so that it's saved even if you haven't submitted the form (which is the mostly likely use case).