Integrate CMake with <catch2/catch.hpp> include paths

This also goes for pkg-config installed by our CMake installation.

This includes

* Updating CMake version on Travis
* Adding a `Catch2` subfolder to the `single_include/` folder to
provide this include path both _inside_ the repository, and _outside_.
* Updated examples to build with the new paths
* Other general CMake cleanup

.travis.yml

@@ -249,7 +249,7 @@ install:
   - mkdir -p ${DEPS_DIR} && cd ${DEPS_DIR}
   - |
     if [[ "${TRAVIS_OS_NAME}" == "linux" ]]; then
-      CMAKE_URL="http://www.cmake.org/files/v3.3/cmake-3.3.2-Linux-x86_64.tar.gz"
+      CMAKE_URL="http://www.cmake.org/files/v3.5/cmake-3.5.2-Linux-x86_64.tar.gz"
       mkdir cmake && travis_retry wget --no-check-certificate --quiet -O - ${CMAKE_URL} | tar --strip-components=1 -xz -C cmake
       export PATH=${DEPS_DIR}/cmake/bin:${PATH}
     elif [[ "${TRAVIS_OS_NAME}" == "osx" ]]; then