I would like to send a PR that does the following:

1. Move the headers from include/*.h into include/midifile/*.h
2. Change code to include as e.g. #include "midifile/MidiEvent.h"
3. Change CMakeLists.txt file to to use target_include_directories in accordance with modern CMake style.

This will make the project more modular and friendly when used as a submodule in a larger project and is in accordance with standard procedure. Do you have any issues with this? If not then let me know and I will send a pull request. Thanks