Allow setting the shadow style from CSS

[Danielle Foré]

To allow for more flexible theming (including possible styles for the critical state), Pantheon-notify should allow setting the shadow style from the theme. Following the syntax for box-shadow should be good. You would get a string like:

-notify-shadow: 0px 2px 18px alpha (#000, 0.60);

Where:
"0px"is horizontal displacement
"2px" is vertical displacement
"18px" is shadow blur
"alpha (#000, 0.60)" is shadow color