Add test case for feature request #3

- Added a preliminary test case for feature request #3.
Create "LimitTrait" for reusability.

- The "limit" parameter is used by multiple API methods, but not all of
  them. Therefore, it would make sense to separate that method into its
  own trait that can then be implemented as needed.
Update `DefinitionsRequest` to use `LimitTrait`.

- Updated the DefinitionsRequest class; removed the existing `limit`
  implementation and made use of the new LimitTrait implementation and
  LimitableInterface.
Implement feature #3

- Created the `AudioRequest` class to fullfill feature request #3.
- Updated the Client, adding the necessary method to create an
  `AudioRequest`. Added the necessary test case.
- Updated the Client, switching to "late-namespace-binding".
Merge pull request #4 from alanly/develop

Closes #3 - word:audio