filename is now optional. Closes #414.

'put' method signature for MediaStorage base class has been
updated. 'filemame' is now optional.
Querying a collection always returns pagination meta.

For consistency, pagination meta is now always returned, even when no
data is being returned, like this:

    "_meta": {
        "total": 0,
        "max_results": 25,
        "page": 1
    }

Performance has been also improved if both HATEOAS and Pagination are
enabled.

Closes #415.