make solver_mode an enum with CPU and GPU -- fully backwards compatible
with old 0/1 style
Merge pull request #403 from jeffdonahue/solver-mode-enum

Make solver_mode an enum: CPU or GPU
fix Makefile bug - HXX_SRCS was things that don't end in .hpp, instead
of things that do...
Merge pull request #406 from jeffdonahue/makefile-include-bug

Fix Makefile header dependency bug