Make the Accept header filter configurable.

Signed-off-by: Michael Gooden <michael@bluepointweb.com>
Merge pull request #13 from MichaelGooden/hotfix/accept-header-filter

Make the Accept header filter configurable.
Use dependency injection instead of service location

- In ApiProblemListener:
  - Created an $acceptFilter property with a default value
  - Created a constructor that accepts an optional accept filter value;
    if a string and non-empty, it will be used to set the $acceptFilter
  - Modified onRender() to use the $acceptFilter value instead of
    pulling from the service locator
- Created a factory in Module for ApiProblemListener that pulls the
  $acceptFilter value from configuration, if found.
- Removed the invokable for ApiProblemListener from the configuration
Merge branch 'hotfix/13'

Close #13
Fixes #9