Add support for filtering in Search endpoint

[ffont]

The search endpoint misses important functionalities like the ability to filter results from third party content providers. Adding support for filters is necessary before making the first public release of the Audio Commons Mediator.

[ffont]

Basic support for filtering was been introduced in (huge) commit faf0547. Nevertheless a number of things are still missing:

• Document filtering functions (left TODOs across the code)
• Add examples in documentation (and API documentation)
• Implement support for more filters in Freesound and in other services