implement makeCacheMatrix and cacheSolve

- add documentation
- try the stuff interactively:
> source("cachematrix.R")
> x =matrix(runif(512*512),512,512)
> xc = makeCacheMatrix(x)
> system.time(cacheSolve(xc))
   user  system elapsed
  0.363   0.001   0.364
> system.time(cacheSolve(xc))
getting cached data
   user  system elapsed
  0.001   0.000   0.001
> system.time(cacheSolve(xc))
getting cached data
   user  system elapsed
      0       0       0
> xc$setinverse(NULL)
> system.time(cacheSolve(xc))
   user  system elapsed
  0.342   0.002   0.344
> system.time(cacheSolve(xc))
getting cached data
   user  system elapsed
  0.001   0.000   0.001