Reimplement LVAlert listener using SleekXMPP

This is an attempt to fix the mysterious subscription losses. This might be a good move anyway because:

• SleekXMPP is better documented than PyXMPP2.
• SleekXMPP has first-class pubsub support; PyXMPP2 does not.

Most of the client implementation lives in the new sleek-lvalert package: see the project page, documentation, and PyPI page.