convenient posterior simulation

Summary:

Add a function to rstan which enables seamless simulation from the posterior draws.

Description:

Its a bit cumbersome to evaluate a function per draw of the posterior at the moment. I am proposing to include a function which takes away lots of the usual boiler plate code needed usually.

Reproducible Steps:

The the attached files. The basic syntax to simulate over a posterior with some function calc_mean will then be:

## the posterior simulate function is clever enough to infer from the
## R function definition of calc_mean what it needs and it will
## automatically grab alpha and beta from the posterior and x from the
## global NS
post_sim1 <- posterior_simulate(post, calc_mean)

Current Output:

If applicable, any relevant output from RStan.
posterior_simulate_example.zip

Expected Output:

If applicable, the output you expected from RStan.

RStan Version:

2.14

R Version:

3.2.3

Operating System:

RHEL 6.7

Begin with the attached posterior_simulate_example.zip and the proposed posterior_simulate(post, calc_mean) call. Use that example to define the supported posterior-draw simulation behavior and verify that the resulting function removes the described per-draw boilerplate; no repository file or test is named in the issue.