Add custom version of selectize.js to work around Firefox bug

This is a workaround for the second issue in #550 found by @tareefk. This version is based on selectize version 0.9.1, and incorporates the fix from brianreavis/selectize.js#521. That fix hasn't been merged into the main selectize repository, but once it is we can update to that version of selectize.

To test this, run:

```R
devtools::install_github('wch/shiny@hidden-selectize')
```

The diff is pretty minimal, so the chances of unexpected side effects are low. A  `git diff --color-words=.` shows that these are the only changes:
![screenshot from 2014-07-24 15 39 26](https://cloud.githubusercontent.com/assets/86978/3694514/1d22bef4-1373-11e4-8cea-58a6192e0baa.png)