Search is a motion

This works, but I definitely need some feedback and some help on writing tests.

Implements `/abc`, `d/abc`, and `2d/abc`. `n`, `N`, and `?` are fairly straightforward if this were to land, but I wanted to keep the pull small for now.

Testing is nutty. I made a little `command-mode-input-field` that you can type in and all is well in the editor. I think I could manage to focus on it and use `textInput` events to simulate typing, but I'm not sure how to simulate enter. But that doesn't matter, because I don't have a `pane` in the test suite, and that's where the little bar needs to be added, near as I can tell, so it just fails right off the bat. 

The search bar looks like this:

![vim-command-input-view coffee - _users_ben_ atom_packages_vim-mode-1](https://f.cloud.github.com/assets/12676/1358734/81036b86-37cc-11e3-9587-f145e9b78369.png)