Extend /incidents endpoint to handle POST requests as well

[yhabteab]

This PR extends the existing /incidents HTTP handler to allow POST requests as described in #452 as well. Also, both the GET and POST handler requires a filter parameter as described in the referenced issue. The changes to the endpoint are trivial but the PR is just that big simply because of the new documentation explaining the new behavior of both HTTP methods and also unit tests for the query string filter parsing and evaluating.

resolves #452

[yhabteab]

Sorry, I've dropped two superfluous if statements.

[yhabteab]

I've changed the JSON decoder for the POST handler to reject unknown fields.

[yhabteab]

While testing Icinga/icingadb#1145, I noticed that using just the NOT EXISTS JSON filter syntax without any other postive matches caused to yield all incidents, which is completely wrong as that would defeat the whole the user must know what he's looking for argument from Johannes. So, the endpoint will now reject the request with 400 if the provided filter contains only negated conditions and documented it accordingly.

[yhabteab]

Sorry, the latest two pushes weren't intentional! I accidentally pushed something here that's supposed to go to #460, so dropped the corresponding changes again.

[yhabteab]

Reduced the PR by ~20 LOC by squashing the some test cases into a loop :).

[oxzi]

LGTM, thanks. Only one small nitpicks where the docs diverge from the code. Please change it, and than it's good for me!