Search/filter tree by file and directory name

Adds the ability to quickly filter/search files and directories by name, using a search input placed on top of the sidebar. Results are displayed live, in pure javascript:

![Selection_002](https://f.cloud.github.com/assets/382538/206109/c6d26e74-81c3-11e2-869c-805b4086c34e.png)

In addition to that, it's possible to (without any specific code) navigate the results list using TAB. I've added some visual cues to help with this:

![Selection_003](https://f.cloud.github.com/assets/382538/206110/e67d6558-81c3-11e2-8093-818837fe6dd8.png)

Using the ENTER key will either navigate to a page, or expand a folder in the tree (and then its sub-items can also be navigated with TAB, seamlessly).

Erasing the text input, or clicking the `x` will clear the results and revert to the original tree.