PROTON-536: Allow find_package(Proton) to work
Export the idiomatic ProtonConfig.cmake and ProtonConfigVersion.cmake
files to allow other cmake projects to easily detect proton.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1577978 13f79535-47bb-0310-9956-ffa450edef68