Paul Mineiro's hypersearch for general usage + enhancements + bug fix

This is a bug-fixed + enhanced + friendlier version of Paul's 'hypersearch' utility in the demo directory.

It can be used to quickly find optimal hyper-parameters by passing a lower-bound, upper-bound, and optionally tolerance (tau) to any vw command where the (single) hyper parameter value is replaced by the placeholder char: %

It uses golden-section minimum search from numerical recipes on vw average loss and converges pretty quickly.

Call 'vw-hypersearch' without args (or otherwise incorrectly) for a full usage message.