Modularized code, support for multiple css files

Moved night check to its own function, added support for multiple
css files instead of just one.

Created toggle function that can be called if user wants to remove
night styles (or apply them in daytime).

Also changed document.getElementsByTagName('head') to document.head
it removes IE8 support according to MDN
Moved variable declaration in toggle

JSHint is much relaxed compared to jshint
Merge pull request #6 from aivot-on/master

Add support for toggling styles