Support Icinga2 hostgroups Tests of hostgroup operations are done in TestFilter

doc: correct doc comment about type

Generate CRUD code using go generate Explanation for use is in the README

Merge Hosts and FilterHosts Getting all Hosts is the same as calling FilterHosts with an empty filter anyway.

Merge get and getFilter methods Doing a GET without a query is the same as doing a GET with an empty query, so there's no point having 2 separate methods.

Merge filter and all objects methods Getting all objects is the same as getting objects with a zero filter.

Deduplicate case conditions Encoding a Host, Service and User as JSON is the same so there's no point having different statements.

I learned how to do some generic programming New type "object" is an interface to represent an Icinga2 object. I added heaps of other stuff! On our way to 1000 lines of code!