Add generic drop-downs

[abitdodgy]

More of a feature request. It would be really nice if there were generic drop-downs. They would be useful for options and configuration buttons/cog wheels.

Instead of attaching them to tabs/nav, they could also be using for small buttons a la Google +

[mkchandler]

This is already in the works, check out pull request #192. I think all of the details are still being hashed out, and it has not made it in to a release yet.

[mdo]

Somewhat of a duplicate, so I'll leave this open for now. Issue #192 is just for buttons, but this calls for the ability to add a dropdown to anything. Not sure if we can do everything, but we can try to do more.

[mdo]

Dropdowns can now be added to anything, but out of the box we don't support everything. The code is separate and you should only really need to add some classes and trigger them via JS for them to work.

We're still looking at adding @kasperp's branch into our codebase for button support. That means we have dropdowns on tabs and navbar links, and hopefully buttons for 2.0.