Improved formatting of Date objects in inspector

Proposed fix for issue #261.

This calls Date.toString and appends it to the description. This is similar to what Chrome does: https://bugs.webkit.org/show_bug.cgi?id=71605

Result:

![fix](https://f.cloud.github.com/assets/766698/1796579/556a3260-6a77-11e3-8f25-61897e80f1b7.png)