Add a makefile and rename main.c to waveform.c

Create a makefile. Add a default compilation rule. This makes it possible to
compile waveform by executing `make`.

Rename `main.c` to `waveform.c`. This allows us to depend upon make's implicit
compilation rules rather than explicitly writing a compilation rule. Adjust
`binding.gyp` accordingly.

Drop some trailing whitespace from `index.js` and `waveform.c`. Make git ignore
`/waveform`, the executable produced by make.