Fix memory leak during parser callback

Part 1: properly use forwarding

Fix/basic json conversion

fixed misprinted condition detected by PVS Studio.

Before this patch, `basic_json` types with different template arguments
were treated as `CompatibleArrayType`. Which sometimes leads to recursive
calls and stack overflows.

This patch adds a constructor and a `get` overload to deal with
different `basic_json` types.

Make integration section concise

Fix constraints on from_json(CompatibleArrayType)

Fixes #924

Make coveralls watch the include folder

Add a note about installing the library with cget