Implement a PSR-4 autoloader

- Creates an extension of the ZF2 StandardAutoloader that does not translate
  underscores to directory separators; as such, it now implements PSR-4.
  Additionally, this will allow underscores in both API and service names.
Merge branch 'feature/psr-4-autoloader'

Implements a PSR-4 autoloader, allowing end-users to use underscores in both API
and service names.