Allow apps specified custom `Zend\Authentication\Adapter\Http\ResolverInterface`

Now apps can specified its own resolver providing a custom 'ZF\MvcAuth\Authentication\AuthHttpAdapter' factory.

Developers can inherited from `ZF\MvcAuth\Factory\DefaultAuthHttpAdapterFactory` and call `setDigestResolver` and `setBasicResolver` methods to specified theirs own resolver.

Configured file resolver still have precedence.
Merge pull request #14 from gsc-frank/allow-custom-http-resolver

Allow apps specified custom `Zend\Authentication\Adapter\Http\ResolverInterface`
[#14] Minor refactoring

- Return `false` early from DefaultAuthHttpAdapterFactory::createService when
  either the config service is missing, or does not have HTTP authentication
  adapter configuration.
- Move the configuration of the Basic and Digest resolvers into
  DefaultAuthHttpAdapterFactory::createService.
- Simplify DefaultAuthenticationListenerFactory:
    - rename createHttpAdapterFromConfig to retrieveHttpAdapter
        - remove code for injecting basic/digest resolvers
    - rename createOauth2ServerFromConfig to createOAuth2Server
        - check for $services->has('config') in this method, not calling method
    - call each of retrieveHttpAdapter and createOAuth2Server in createService,
      injecting the AuthenticationService only if they return something
- Ensured all tests are passing.
Merge branch 'hotfix/14'

Close #14