Add basic UI to S3FS

This PR closes #15.

This will just create a very basic initial working implementation of the UI.

Once this is merged, enhancements can be made via future PRs:

- Validation of the credentials using [AWSValidator][]
- Polymer components for the input fields, button and toasts using [ToastManager][]

[awsvalidator]: https://github.com/lavelle/chromeos-filesystems/blob/s3fs/s3fs/ui/js/lib/awsvalidator.js
[toastmanager]: https://github.com/google/chromeos-filesystems/pull/10

Current status:

- [x] Only support one instance per provider
- [x] Open a window prompting the user to enter credentials when the extension is installed
- [ ] If the user enters invalid credentials, or any other error occurs, the window will reappear with an appropriate error message, until the user enters correct credentials or closes the window
- [ ] If the user closes this window, the extension goes into a deactivated state
- [x] The filesystem instance persists as long as the extension is installed, and remounts itself after any Chrome relaunches
- [x] To unmount the filesystem instance, the user uninstalls the extension
- [ ] If the credentials ever expire or change, the extension will go into a deactivated state
- [ ] When in the deactivated state caused by 4 or 7, the extension will prompt the user to enter new credentials as in step 2 whenever an attempt is made to use it
but i want to check that it's not the empty string either. this covers all falsy values