Bing key overhaul

- Use a "Basic" key by default in `BingMapsImageryProvider`.  If this default key is used, write a message to `console.log` at construction time informing the user that they should generate their own as soon as possible.
- Add a new property, `BingMapsImageryProvider.defaultKey`.  Users can set this once rather than passing the key to the `BingMapsImageryProvider` constructor.  This saves the hassle of passing the key down through the `Viewer`, `CesiumWidget`, etc.  If a default key is specified, the message described above is _not_ written to `console.log`.
- Output messages written with `console.warn` to the Sandcastle console.
- In Sandcastle, don't switch to the Console tab automatically for `console.log` messages.  Instead, show the number of new messages as a number in parenthesis after the tab name (ie: _Console (2)_).  However, we still switch automatically for other types of messages, including exceptions, `console.error` and `console.warn`.