[UX] Issue #4174 - Add find next / previous buttons for easy navigation

Add next/previous buttons to Find modal bar for easy navigation between matches.

Handles cases
1. Find has matching results
1. Find has no matching results
2. Large documents

![screen shot 2013-08-29 at 5 38 26 pm png](https://f.cloud.github.com/assets/1189142/1054576/aadeb6b8-110c-11e3-8056-c7f4f5de1cea.jpg)