Added "make install" support to the CMake project.

- Does everything as the autoconf script does (I think).
- Generates SSL certs for the test-server
- Installs all test apps.
- Defaults to /usr/local/
- "DESTDIR=/bla make install" works
- Append "libwebsockets" to start of the test-app names like the autoconf stuff does.
- Only tested on OSX so far...
Create the "doc" dir before generating docs.

The documentation wouldn't be generated if the path didn't exist.