Tweaks to run on JDL dev env
Failing specs for List#shift and List#unshift
Implemented List#shift, #unshift, #clear
Replaced << in initialize with unshift

Should be O(n) instead of O(n^2/2). Also confirms behavior of unshift,
and led to recognition that unshift needs a second optional arg: the subject of
the new element.

Note: there's a slight divergence in behavior: creating an new empty
list with an explicit subject won't "unshift" into that node.
Move Judson from AUTHORS to CREDITS. Added to Contributors in README.