Allow construction of stan::mcmc::chains using vectors (rather than Eigen)

Added to make wrapping Stan functionality easier for other software, e.g.,
PyStan.
Merge pull request #615 from ariddell/feature/chains-adapter-pystan

Allow construction of stan::mcmc::chains using vectors (rather than Eigen)
Merge branch 'develop' into feature/cmake
Building the Stan reference manual now works.