added the ability to set z-index of the notification

This helps with making sure the it appears above other elements which
have a custom z-index set.
added the ability to specify an offset for the notification

This allows one to configure the notification to position it further
from the edge of the screen incase it needs to clear toolbars et al
added the ability to have the notification appear at the bottom center

This is done by setting the position to "bottom".  Could be useful to do
some more work to allow bottom right/bottom left in future.
Merge pull request #10 from AutogrowSystems/useful-additions

Useful additions (Thanks for the support)