(maint) Install facter header files.

Adding facter header files to install target.
This allows users to build against libfacter from a C++11 program.
Updating README with an example program.
Deleting vender README as it's outdated and the information is present
in the root README.
Merge pull request #41 from peterhuene/maint/installation

(maint) Install facter header files.