valueForMirror in autogrow-textarea now replaces spaces with &nbsp;

If a line starts with a space, the mirror does not offset properly.

![screen shot 2014-11-23 at 4 02 52 pm](https://cloud.githubusercontent.com/assets/916259/5159840/39d242f6-732a-11e4-9812-b9e6810f26b9.png)
![screen shot 2014-11-23 at 4 03 05 pm](https://cloud.githubusercontent.com/assets/916259/5159839/39ce1910-732a-11e4-84d1-bb426b6eea08.png)

This modifies the valueForMirror function to replace spaces with `&nbsp;`.