Re-factor spyalg code layout to create a separate module from the thresholding functions
for the iterative soft thresholding algorithms. Also begin populating the package
namespace. All code that worked with the previous version will need to be modified to
work with the new layout.
Complete rewrite of iterative soft thresholding algorithms to be based on the more
general proximal gradient descent algorithms. Implements 4 types of prox algorithms
(gradient descent, accelerated gradient descent, ADMM, and linearized ADMM). Includes
formulations for standard problems like l1-regularized least squares and basis pursuit
with denoising.

The AMP algorithms are still a work in progress and do not work correctly (as far as I
know) at this moment.