cosmetics for Weibull
add Frechet distribution

based on Weibull code
add function signatures tests for Frechet distribution
add _rng test for Frechet distribution
merge changes from alyst
Revert "merge changes from alyst"

This reverts commit e5e6ed5284676d8a05f775785487d8a3b61b72b3.
merge frechet
moved distribution function signature test
Merge branch 'develop' into feature/frechet-distr
added frechet_rng test against quantiles generated in R
Merge branch 'develop' into feature/frechet-distr
remove unused code and fixed frechet test
Merge branch 'develop' into feature/frechet-distr
merge develop
switch to check_positive_finite in frechet distr file
fixed lower bound test for frechet distr
Merge pull request #807 from stan-dev/feature/frechet-distr

Feature/frechet distr
document Frechet and Weibull distributions relation
Merge pull request #825 from alyst/doc-frechet-weibull

document Frechet and Weibull distributions relation
Merge branch 'develop' of https://github.com/stan-dev/stan into feature/issue-771-ode-solver