Bugfix. Modal height wasn't being calculated correctly.

The functions setPosition and measure didn't have access to $el on which the
pattern was applicable.

Instead they used jQuery to get the element. The problem occurs when there
are multiple div.pat-modal elements in the page. Then there's no way to know
which one is the right one.

The fix is the set the correct $el element on the pattern and to access that in
setPosition and measue. "measure" is called asynchronously, so we have to use $.proxy to
make sure it has access to the pattern object.