Remove ActiveSupport Dependency

Whereas:
- ActiveSupport has a big footprint.
- Requiring it can affect compatibility with other libraries
- The parts of it used in rbp are minimal

Remove ActiveSupport in favor of custom monkey-patches.
Adding the Inflecto gem as a dependency as it was extracted from
the ActiveSupport Inflector and alows rbp to nicely handle
pluralizing and singularizing strings.

Rbp will fallback on its custom patches if ActiveSupport is not
available