generate:bundle Allowing a namespace to be used without a vendor namespace

Hey guys!

This is a quick, BC version of #312, which has become a major update to the entire command. That's great, but it will break some BC and will need to be released as a new version.

But right now, it's still impossible to generate a bundle without a vendor namespace. That was the original issue, and the fix is very simple. This fixes that in a BC way. If you *do* omit the vendor namespace, it even gives you a warning (in case someone accidentally says `Acme/FooBundle`, which looks like `AcmeFooBundle` to the system).

I've tested both sides of the flow and it works fine. If someone else can double-check, that'll make Fabien's merge much easier. Screenshots Below!

Thanks!

![screen shot 2014-11-25 at 5 16 28 pm](https://cloud.githubusercontent.com/assets/121003/5192677/59c114c2-74c7-11e4-9f0e-d4a36366598c.png)
![screen shot 2014-11-25 at 5 16 51 pm](https://cloud.githubusercontent.com/assets/121003/5192676/59c0e0e2-74c7-11e4-9e77-8ade8dc48ac1.png)