set next good fft size to 2 for 0 to avoid infinite loop
adding test for autocorrelation
Merge pull request #134 from stan-dev/feature/rstan_issue8

set next good fft size to 2 for 0 to avoid infinite loop
initializing local variables fixes issue 91; still need automated test
Merge branch 'develop' of https://github.com/stan-dev/stan into feature/issue-91