Remove use of 'register' keyword

The 'register' keyword is not used by modern compilers and it is
depracted in C++11.  When compiling with clang++ and the -std=c++11
flag the depracated keyword triggers a warning which causes compilation
to fail since warnings are treated as errors.
Merge pull request #3092 from southerngs/remove_register_keyword

Remove use of 'register' keyword