Merge branch 'develop' of https://github.com/stan-dev/stan into feature/issue-1065-index-traits
type trait metaprogram for index types added and tested
added value_type metaprogram with tests
replaced Eigen add-on for value type with metaprogram
removed EigenDenseBaseAddons and replaced usages with metaprograms index_type and value_type
removed ::size_type from distribution test utility
Merge pull request #1075 from stan-dev/feature/issue-1065-index-traits

Fixes #1065. Feature/issue 1065 index traits