make CSS transitions work properly

CSS transitions work fine when the modal is sliding into view, but when the user clicks away, the modal just disappears instead of sliding out of view.

This mainly had to do with:
1) the omission of bootstrap-transition.js (bootstrap modal will not attempt to hideWithTransition if $.support.transition is undefined)
2) mixin CSS treatment to the fade class causing it to have opacity 0
3) the modal having a "fade-large" class instead of "fade" in some cases (instagram, dribbble). bootstrap-modal.js looks for the modal to have a "fade" class before calling hideWithTransition()

I only tested this last issue on Instagram because I do not have a Dribbble account.