Add straighten operation and action

This pull request adds an operation and action to straighten a way. This addresses https://github.com/systemed/iD/issues/726 and is modeled after the behavior found in Potlatch2.

![screen shot 2013-09-19 at 1 07 47 pm](https://f.cloud.github.com/assets/366320/1176179/362b3f42-2167-11e3-8f1d-e877fd64e3c3.png)