Consolidate core dotfiles code into bin/dotfiles.

* source 00_dotfiles.sh for useful dotfiles script vars/functions
* homebrew-specific functions are defined in the homebrew init script
* tests have moved to the top-level test folder
Make ubuntu sudoers message more accurate.
$DOTFILES env var points to dotfiles root dir.
Further consolidate dotfiles code.
Change cached selected init scripts filename.
Improve assert function.
improve path_remove function, add tests.
Update copyright year. As if it matters.