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.