removed reference to GM_log as it is deprecated

GM_log is deprecated. The usage of it causes errors in Tampermonkey on Chrome.
Better logger selection 

Choose the logger in this order:
1 console.log
2 GM_log
3 window.opera.postError
4 function(){}
fixed console and GM_log detection

so that not everything will end up in a nasty `ReferenceError`
Update gm_config.js

isGM it is
Rewire the logic for choosing what to use for logging. Closes #59.
Merge branch 'logging'

Conflicts:
	gm_config.js
Merge pull request #61 from sizzlemctwizzle/logging

Rewire the logic for choosing what to use for logging. Closes #59.
Confirmed working in current Chrome/Chromium/Opera.