Change name of backdrop class and fix selector-bug

Change the backdrop class' name to a more generic one and
fix the hardcoded trigger-selector to actually reflect the
user's option instead of overriding it with always the same
class '.trigger'.
Supply option for color and opacity

Color and opacity of the backdrop have been externalized into their own option
and both should be supplied as strings:
   backdropOpacity : String, "0.7"
   backdropColor : String, rgb-formatted, "255, 0, 0"
Merge branch 'master' of github.com:baumanno/jquery-modal

Conflicts:
	js/modal.js