cachematrix:  Implement a matrix that can cache its inverse.

Implements a CacheMatrix--a matrix that uses R's lexical
scoping rules to cache its inverse.

Implemented two functions:
makeCacheMatrix:  Convert a matrix into a CacheMatrix that can
                  cache its inverse.

cacheSolve:  Calculate or retrieve the inverse of a CacheMatrix

Test case:  Created an invertible 2x2 matrix x.  Created a
            CacheMatrix cacheX with makeCacheMatrix(x).
            Calculated inverse with cacheSolve(cacheX) and
            saw that message indicating cache hit was not
            displayed.  Calculated inverse with
            cacheSolve(cacheX) and saw that message indicating
            cache hit was displayed.  Verified inverse by
            observing cacheSolve(cacheX) %*% cacheX[['getValue']]()
            evaluates to the identity matrix.