Strings needed for Edge Code "default editor" feature

User story is currently in progress and _may_ land before Brackets Sprint 30 wraps up, so we need to get the strings in today to keep that option open.

@njx Want to review since you already looked at the string?

The dialog will look like this:
![file assoc dialog](https://f.cloud.github.com/assets/1197510/1018950/d53b8b2e-0c35-11e3-8e3d-69395fe4c954.png)

(CC @iwehrman @bchintx)