added rayleigh distribution with tests and doc in manual
removed extra manual file
fixed headers in manual for rayleigh distribution
decreased number of decimal places in rayleigh tests
used OperandsAndPartials for gradients of cdf for rayleigh distr
added missing using statement and missing rng test for rayleigh distr
Merge pull request #103 from stan-dev/feature/add_rayleigh_distr

Feature/add rayleigh distr