makefile fixes: setting DEPFLAGS_OS only if it hasn't been set by the user; adds O option to building Sundials

Fixes #954. Cleaning up makefiles. 

This reverts commit 34f47e34.

This reverts commit 5fb6d7c9.

This reverts commit dc7d76ae.

Remove argument decorators from GPU kernel signatures

Feature/issue 970 different return types for adj_jac_apply

Change from c++11 to c++14 in README

I tried to compile the simple example cpp script presented in README and got an error that `std::index_sequence` couldn't be found. The fix was to use `-std=c++14` instead of `-std=c++11`.

Fix the GPU tests and re-merge GPU matrix multiply

Feature/issue 1018 beta proportion log functions

enable if the dot product kernel

# Conflicts:
#	stan/math/gpu/multiply.hpp

Also check that input matrices have size rather than they have nonzero rows x cols