This simplifies the project structure by making the following changes:
1. The file/directory hierarchy now matches the pattern used for most
Ruby projects. This means no more init.rb, and all code now resides
in lib/release_tools/ instead of lib/.
2. All classes and modules are namespaced to the ReleaseTools module,
instead of them being defined at the top level.
3. Dependencies are now loaded using regular require calls in
lib/release_tools.rb, instead of relying on a mixture of require,
require_relative, and autoload.
4. The colorize Gem is updated to support the use of frozen strings, and
because the used version was from 2009.