Build system

Add automatic library detection, easy integration and other fancy stuff to the build system. I haven't tested it thoroughly, but there shouldn't be any major bugs.

Integrating backward in an existing CMake project:
```
add_subdirectory(/path/to/backward-cpp)
add_executable(mytarget example.cpp ${backward_ENABLE})
add_backward(mytarget)
```

If you're fine with the changes, I can also update the README.