Fix indent guide

This fixes the indent guide when lines around the whitespace lines change. It will check whitespace lines surrounding a change. If their indent level changes, it will invalidate the whitespace lines.

Fixes this issue:

![ba4df5ea-f898-11e3-9fcd-686fe9e0ea2c](https://cloud.githubusercontent.com/assets/69169/3378927/0bd7fa88-fbea-11e3-8427-0256ec8fa576.png)

@nathansobo will you check the changes in tokenized-buffer and make sure it's legit? Also, let me know if there are benchmarks I can run to make sure typing is ok.

ref #2367
should this be broken into two specs?
"updates the indent guides on empty lines preceding an indentation change"
"updates the indent guides on empty lines following an indentation change"