Added libbe.command.serve and libbe.storage.http for HTTP backend.

Added libbe.command.serve and libbe.storage.http for HTTP backend.

Now the following works:
  some-BE-dir$ ./be serve
  $ ./be --repo http://localhost:8000 list

I haven't come up with a clean idea for testing this yet, so other
commands may be broken, but once we get the testing working, it
shouldn't be too hard to get everything working over HTTP :).