update NEWS file
bump version to 2.1.0
update stan submodule
wrongly try to get rid of one of R CMD check warnings

register functions implemented in cpp
stan submodule uses v2.1.0 tag
try to register C routines, which is not necessary
finishing registering C routines
Merge branch 'develop' of https://github.com/stan-dev/rstan into develop
Merge branch 'develop' into release/v2.1.0
Merge branch 'release/v.2.1.0' into develop