Updated BFGS minimization routine to use gradient-fn to estimate f-prime
when it is not provided. This also entailed changing gradient-fn to
accept matrices as well as vectors.
Fixed so tests pass. f-prime needs to be passed as a keyword argument.
Updated documentation to match new use of f-prime.
Merge pull request #218 from JackSullivan/merge-minimize-f-prime

Updated BFGS minimization routine to use gradient-fn to estimate f-prime