Add support for monitors to indicate they may block

Useful when the network is down or something else is causing issues.
Add a "ready" check to MPD monitors

If the MPD server isn't responsive or the network is down, MPD will no
longer block xmobar from doing its job.
Find out whether the weather monitor is likely to block

Uses the HTTP module to do the work rather than curl.
Drop curl dependency for the HTTP module
Handle errors better in Weather plugin
News update