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.
Merge branch 'cmake_config' of github.com:JoakimSoderberg/jansson into cmake_config

Conflicts:
	CMakeLists.txt
	cmake/JanssonConfig.cmake.in