Lasso regression

Create a lasso regression routine, similar in spirit to `leastSquares`, but with a penalty term for the l1 norm. Goal is to compute sparsified version of least squares solution. 

Depends on #300 
I'm not sure which method you are suggesting I use (`zeros` on what?), but I am happy to port things over to fit the official breeze style (provided performance is unaffected, which it likely would not be in this case).