Cmake config

This is because it's really easy to get a name collission if compiling
Jansson as a subproject in a larger CMake project. If one project includes
several subprojects each having their own config.h, this will cause the
wrong file to be loaded.

This will simplify linking against the lib, both from the build-tree and
install-tree from other CMake projects.

CMakes find_package command uses these configs to locate the exported
targets for the library.

* Also changed so that all CMake options for the project are prepended
  with JANSSON_ so that there is no ambiguity when including this as a
  subdirectory in another CMake project.

Check for key == NULL in json_object_get and json_object_del

Fixes #147.

Fixes #146.

Oops.

paper bag bug: json_string_set* functions should set the length too

fix type mismatch anywhere int != int32 (Vorne)

add new public functions to jansson.def

Support "\u0000" in strings

Also:
  Steal strings during parsing for half the mallocs!
  Change all input-caused assertions to errors.  No crashes please, we're programmers.