Eliminate unused variable in CUDA's reduce_by_key implementation.
Avoid some upper-cased variable names.

Fixes #413
Fixes #391
Merge pull request #415 from jaredhoberock/reduce_by_key-fixes

Eliminate unused variable in CUDA's reduce_by_key implementation.
Avoid inheriting function objects from std::unary_function and std::binary_function.

Fixes #414
Merge pull request #416 from jaredhoberock/functional

Avoid inheriting function objects from std::unary_function and std::bina...