Add middle truncation feature

This adds a new "middle_truncate" option, which if set to true will cause the displayed text for single-selects to be truncated in the center, Mac-style.

Couple of notes:

This was done against v0.9.8, since that is the version I am using.  (Newer versions seem not to work for me.)

I only implemented the middle-truncation logic for jQuery, since I do not use prototype.js.

Has not been tested in IE.