Reorganize bash profile configuration

.bash_profile now only sources other scripts which have all of the
actual configuration.
.profile is still used for shell-agnostic configuration, and is now
overridden by .profile_local if that exists.
.bashrc is used for bash-specific configuration, and is now overridden
by .bash_local if that exists.
.bash_osx was added which holds OS X-specific bash configuration, and
should be sourced by .bash_local on any OS X machine.