Add output writer for onsets. Added test data.

Added writer interface as well as graph_writer implementation to onsets. Graph
writer writes out a newline seperated file where each value represents one
sample. A 1 at the value means an onset was found at that sample while a 0
means that no onset was found.

Added new /data/ folder containing our test data.  The onsets program has also
been run over this test data, which produced the *.out files (yes, I know, a
better file extension is sorely needed).