calculate the number of workers based on the cpus

The number of workers should be 2 * the number of CPUs in the system
by default. This allows us to have parity with the value used in
production (hardcoded to 8, since we're using 4-cpu systems across the
board).
Merge pull request #40 from opscode-cookbooks/jtimberman/unicorn-worker-by-cpu

calculate the number of workers based on the cpus