Calculate tile size.
Add stacked/alpha options.
Update README and move app.config.js examples
Handle prime numbers.

Prime numbers suck (in this situation) because they only allow two
possible tile arrangements, and they both usually suck, for common
window sizes. (E.g. 1 row of 13 columns or 13 rows of 1 column.)

This tests for primality (only one set of factors) and if it's prime and
not 2 (you're screwed with 2) it finds the best size again for n + 1
tiles. If that's better (it almost always is) it goes with that.
Essentially we pad a blank square. We could do something fun in that
square if we wanted to.

Most primes are surrounded by more factorable numbers. It works in the
common cases.