Fix default scaling for squareroot

Without this, the scaling is thrown off when the block doesn't have a height yet (not entirely sure why this is happening, but we're using Mathquill from within other JavaScript that's rendering it's containers)
Fix issues with mouse event handlers when in an iframe

Using Mathquill within a TinyMCE plugin iframe window, the following steps would make it blow up:

1. Put some text in the Mathquill textbox
2. Click and drag to select the text, moving your cursor outside the window
3. Press backspace
4. Move your mouse back on top of the window

This fixes the issue by listening for the mouseup on the parent document and unbinding listeners appropriately.
Merge pull request #2 from foraker/iframe-fix

Iframe fix