Xcode support

Nobody has claimed this yet.

Many developers at Mapbox have come to rely on the XCode IDE for developing C++ on OS X. It offers some amazing productivity enhancers like code completion, code folding, a built-in GUI for static analysis and address sanitizers, and more. This is why https://github.com/mapbox/node-cpp-skel/pull/13 was added to node-cpp-skel.

For hpp-skel the support will be much easier to add since we are using `cmake` here and `cmake` can autogenerate xcode files. Mirroring node-cpp-skel support will be as easy as adding:

```diff
diff --git a/CMakeLists.txt b/CMakeLists.txt
index a81a292..aa0f8ce 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -12,4 +12,4 @@ include_directories(SYSTEM ${MASON_PACKAGE_catch_INCLUDE_DIRS})

include_directories("${PROJECT_SOURCE_DIR}/include")

-add_executable(unit-tests test/test.cpp)
\ No newline at end of file
+add_executable(unit-tests ${PROJECT_SOURCE_DIR}/include test/test.cpp)
\ No newline at end of file
diff --git a/Makefile b/Makefile
index ad76d68..89b2584 100644
--- a/Makefile
+++ b/Makefile
@@ -9,6 +9,9 @@ debug:
test: default
./build/unit-tests

+xcode:
+ rm -rf build/* && mkdir -p build && cd build && cmake ../ -G Xcode && open *xcodeproj
+
coverage:
./scripts/coverage.sh
```

The change of `+add_executable(unit-tests ${PROJECT_SOURCE_DIR}/include test/test.cpp)` gets the headers to show up in XCode (otherwise it does not see them by default).

/cc @kkaefer


Update CMakeLists.txt so headers appear in generated Xcode projects, and add the xcode target to Makefile using CMake's Xcode generator. Start by comparing the proposed changes with node-cpp-skel support, then run the xcode target and confirm that an .xcodeproj is generated and opens successfully.

Written by the indexing model from the issue text.