Use openssl's cryptographically secure RNG

So simply going straight to openssl seems to be the recommend course of action in a number of R packages dealing with cryptographically insecure RNG (e.g. [randaes](http://cran.r-project.org/web/packages/randaes/index.html)  and [digest](http://cran.r-project.org/web/packages/digest/digest.pdf).

Re #95 I proposed the necessary edits, but they would come at some cost, using openssl via system calls is 224-335 times slower. Maybe the RNG generator could be pickable through a config option? In my case using cryptographically secure keys would really be imperative, but MMV. OpenCPU should maybe default to the more secure version though...

> system.time({
 	for(i in 1:1000) {
 	hash <- paste0("x0", system( paste("openssl rand -hex",64/2), intern = T ) )
 }
 })
##   user  system elapsed 
##  2.530   1.678   4.357 
> characters <- c(0:9, letters[1:6]);
> system.time({
 	for(i in 1:1000) {
 		hash <- paste(c("x0", sample(characters, 32, replace=TRUE)), collapse="")
 	}
 })
##   user  system elapsed 
##  0.012   0.000   0.013 
> 4.357/0.013
## [1] 335.1538