Fix CreatePages

This fixes #450.  There are two problems:

1.) We're creating a new goroutine for every page.
2.) We're calling s.Pages = append(s.Pages, page) inside each goroutine.

1 is a problem if in that if you have a ton of pages, that's a ton of goroutines.  It's not really useful to have more than a few goroutines at a time, and lots can actually make your code much slower, and, evidently, crash.

2 is a problem in that append is not thread safe. Sometimes it returns a new slice with a larger capacity, when the original slice isn't large enough.  This can cause problems if two goroutines do this at the same time.

The solution for 1 is to use a limited number of workers (I chose 2*GOMAXPROCS as a nice guess).
The solution for 2 is to serialize access to s.Pages, which I did by doing it in a single goroutine.
Adjusting the number of go routines to build the pages in to 4*GOMAXPROCS (from 2) for faster rendering.