Add kernel-select dropdown to new notebook button

Depends on #6949

- most recent choice is remembered in localStorage
- kernel can be selected with a url parameter

I didn't add kernel icons because #6537 hasn't been merged. I also didn't remove the kernel selection widget on the notebook page, because I think that can be done as a single PR that addresses #6217.

Kernel selection dropdown:

![screen shot 2014-11-15 at 12 40 35](https://cloud.githubusercontent.com/assets/151929/5059178/d254c262-6cc5-11e4-98ed-94a4e15ddb58.PNG)

New notebook title-text with IPython currently selected:

![screen shot 2014-11-15 at 12 40 49](https://cloud.githubusercontent.com/assets/151929/5059172/bd739ae4-6cc5-11e4-8cf4-6506ebb39e36.PNG)

ping @ellisonbg @fperez

`current_selection` is no longer a very clear name for this - `default_kernelspec`?