Update cachematrix.R

complete the assignment. Test run would look like the following:

x <- rbind(c(1,-1/2),c(-1/2,1))
m <- makeCacheMatrix(x)
x
m$get()
# should return the matrix x
cacheSolve(m)
# should return the inverse but _NO_ "getting cached data" message
cacheSolve(m)
# should return the inverse _AND_ a "getting cached data" message