Implement preconditioning and scale-invariant update rule
Implement preconditioning in fminbox and nnls

This also allows the default parameters to change, since the accuracy
will be higher thanks to preconditioning.
Improve flexibility of display in test function
Test nnls
Update README
Add Kate's temporary files to .gitignore
Simplify regularization of the preconditioner for box constraints
Merge pull request #14 from timholy/master

Add preconditioning and improved descent for conjugate-gradient algorithm