Added DismissListener to be called whenever the Snackbar is dismissed.
Changed in and out animations to follow the Material Design specs. Also renamed
animation resource files to snackbar_in and snackbar_out to make it less likely
for the parent project to accidently override the animations. Naming them
snackbar_in and snackbar_out also allows people to intentionally override them
with custom in and out animations.
Merge pull request #6 from SteadfastInnovation/feature/update_animations

Changed in and out animations to follow the Material Design specs.
Merge pull request #5 from SteadfastInnovation/feature/dismiss_listener

Added DismissListener to be called whenever the Snackbar is dismissed.