Better multi monitor support

[mgurga]

Fixes a problem where fullscreen, monocle, and other functions would only use the first monitor. The commit creates a method that finds which monitor a client is on and passes it to the fullscreen, monocle, and other functions. This PR also partly addresses #18.

[JLErvin]

Hmmmm, this might require a bit of investigation. The purpose of the ws_m_list (which, I think is poorly named and should be more verbose for a variable which has global scope) is to map workspaces onto monitors. Since each client maintains it's active workspace, it should be the case that we can correctly identify which monitor it is on using the list as a lookup. It's likely the case that this is not getting updated properly in some instances - leading to the bug.

I will look into this a bit. I'm quite busy nowadays, however, so if I can't find anything soon I'll just merge this change to be pragmatic.