Added CMake Find_Package script and debug suffix

* This allows easy inclusion of EntityX in CMake projects using
`Find_Package(EntityX)`.
* Also debug versions now use the `-d` suffix, no longer overwriting
release builds (e.g. `entityx-d.lib` rather than `entityx.lib`).
Merge pull request #73 from MarioLiebisch/cmake-additions

Added CMake Find_Package script and debug suffix