Support filter params in HTTP GET request content

[JochenReinhardt]

In order to avoid URL length issues with complex filters, I would like to add an option to pass the filter as HTTP GET request content.

This should be done at least for nested filters (http://www.crnk.io/releases/stable/documentation/#_nested_filtering) as there is a single filter query parameter that actually contains a JSON-formatted object.
I would not mind passing any other filters also in the request content for consistency reasons. As far as I understand, it should be feasible as filters in the URL e.g. GET /tasks?filter[name]=Super task can be written as JSON-object, too:
{ "name": "Super task" }.

As this is non-standard-behavior, I would like to implement a configuration option that is by default turned off. Something like crnk.config.resource.request.filterCriteriaInHttpContent in CrnkProperties.

I am going to have a look at this myself and hopefully will come up with a pull request, soon.

Any comments, help or objections welcome.

[JochenReinhardt]

Opened a feature branch in a forked repository.
@remmeier I really could need some advice if this is going into the right direction.

[JochenReinhardt]

Opened pull request: #771