Adds animateDropbackIn and animateDropbackOut ability to modal

This allows the user to specify any jquery animation for the
backdrop along with any options for that animation
Update readme with animation example
Update readme with animation example
Merge branch 'animation' of github.com:flynfish/ember-bootstrap into animation
Add test to ensure backdrop removed when using animation options
Merge pull request #86 from flynfish/animation

Animation support for modal backdrop