Version 1.56

- a bunch of edits to man pages
- added a logit function for convenience
- uniform priors in map2stan work via constaints now and are commented
out in the model block, where they would be redundant
- added automatic positive constraints for scale paramters in
dbetabinom, dgampois, dgamma2
- fixed a subtle bug with WAIC calculations for map2stan models using
imputation
- Added quick installation section to README