Some cleanup and adding ex1c gradient checker.

- Rename ex1b_load_mnist.py to load_mnist.py since it is used in both
  1b and 1c. Make corresponding changes where necessary.

- Change gradient check solutions to be called from ex1/ directory,
  not from solutions directory directly.  Makes things a bit cleaner.

- Small fix to gradient checker: flatten theta first.

- Add reshape of theta in to softmax_regression template.