Fix for #33

Hey,

This PR fixes Issue #33 in two ways.

1. The reported error message. Now every toast has it's own animator, so that an animator won't have gravitybehaviors for two notifications. This currently isn't entirely necessary, but would be, for my idea of stackable notifications. (see #29).
2. The actual bug. The bug occured, because the first notification set the  gravityAnimationCompletionBlock to NULL, _after_ the next notification set it's own block as the value. The value now is set right before the next notification is displayed, to prevent this.

Hope everything is okay with these changes