Added CMake config files.

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.
Renamed config.h -> jansson_private_config.h

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.
Merge pull request #157 from JoakimSoderberg/cmake_config_new

Cmake config