Mark relevant numeric_limits methods constexpr

This works now that stan is C++11. Previously, there was no
way to do the equivalent with doubles since we could do static
compile-time methods like this for integral and enum types.

Fix linking issue where static members are defined but not initialized

In particular, some members were declared but not defined, which
sometimes causes a linker error. See #806 for more discussion.

changes to opencl_context_rewrite

Revert the matrix autos from #801.

license link in README, fixes #802

Change so that there is a singleton base class and then an class that acts as an API to that base class

Use auto so we can get the benefits of Eigen template expression optimizations

Add Stage in Jenkins to make gh-pages with doxygen

Merge branch 'opencl_context_rewrite' of https://github.com/rok-cesnovar/math into opencl_context_rewrite

This reverts commit a4078ceb.

Feature/issue 760 cvodes refactor fixes #760 

Merge branch 'opencl_context_rewrite' of https://github.com/rok-cesnovar/math into opencl_context_rewrite