Avoid name conflict with UI Bootstrap

Their is a lot of people having troubles with `angular-flexslider` because their is a name conflict with UI Bootstrap.

The quick fix was to simply create a custom build without the carousel. However, I don't think it is always an acceptable solution. When you get all your libs with bower, it is a pain in the neck.

To fix this, I propose to support both `slide` and `flex-slide` as an attribute.

Nothing in the module is changed so far and UI Bootstrap are now happy campers again! :-)