add makeCacheMatrix & cacheSolve to cachematrix.R

Updates to cachematrix.R file to create two functions, makeCacheMatrix
which creates a special 'matrix' object that can cache its inverse, and
cacheSolve which uses as input the special 'matrix' object from
makeCacheMatrix and computes the matrix inverse only if it has not been
previously calculated (or the matrix hasn't changed), otherwise it
returns cached value.