1.4 - imputation arrives (experimental)

- map2stan automatically detects NA in predictor variables and
generates imputation code, when a distribution is assigned to the
predictor. this is still experimental, but is working on test cases.
some post-processing functions may not behave as expected, yet.
- still improving postcheck()
- fixed bug with ordered map2stan template
- improved auto-typing with ordered distributions in map2stan
- added dbeta2 to map2stan templates for p,theta parameterization of
beta distribution