Extensible web plugin

Following discussion in #456 and on the mailing list [here](https://groups.google.com/d/msg/beets-users/lxj6TZ4bsZk/aNozth-gwBAJ) and [here](https://groups.google.com/d/msg/beets-users/Y0U70pAEFVg/nml6KFr7AcMJ), it's time to get slightly more serious about overhauling the web plugin for extensibility.

Here's my wishlist:
- Start with a clean slate REST API. We should design this with several potential use cases in mind. The old API can stick around for backwards compatibility; perhaps the new one goes under `/v2/` or something. The new API should be standards-worthy: clean and well-documented, potentially even for reuse by other tools.
- Design a new Python API for exposing functionality via the web API.
- This probably means rolling the web API into beets core instead of keeping it in a plugin.
- Accordingly, we should separate the UI from the API itself. As I mentioned on the mailing list, this would all work best if beets core could avoid including the frontend HTML/JavaScript/CSS itself; this should be relegated to pluggable client interfaces. We could provide a simple default interface (perhaps hosted somewhere as static files?) but make it easy to switch to others.

How does all that sound to everyone?