Adds cmake package configuration and version file to enable find_package(libpion)

Hi dear splunk developers team,

I was just about to use your library in my project and noticed that your cmake build didn't add package-config, but personally I wished to be able to find_package pion, so I simply added the necessary steps for this.

This allows users of the library to include it easily then :
       find_package(libpion @LIBPION_LIBRARY_VERSION@ REQUIRED)
       link_directories(${LIBPION_LIBRARY_DIRS})
       include_directories(${LIBPION_INCLUDE_DIRS})
       target_link_libraries(yourTarget ${LIBPION_LIBRARIES})

It also has the advantages that if different version are on the system, only the right one is taken. 

Thanks for this http library :smiley: 
Cheers,