add CMake package configuration file

This will enabled other CMake-based projects to find an installed version
of cpp-netlib via

   find_package(cppnetlib [[VERSION] 0.10.1] [REQUIRED])

To link to the installed cppnetlib version projects want to use

   include_directories(${CPPNETLIB_INCLUDE_DIRS})
   target_link_library(myTarget ${CPPNETLIB_LIBRARIES})
Merge pull request #303 from torbjoernk/add-cmake-config

provide cppnetlibConfig.cmake file