Some refactoring and improvements in the fits2png script.
Merge branch 'master' of github.com:nnen/waterfall
Add some unit tests.
Add waterfall_snapshot_length config option.

 * Replace the waterfall_buffer_size config option with
   waterfall_snapshot_length, which sets the length of a snapshot in
   seconds and the buffer size is calculated accordingly.
 * Add Doxyfile so that doxygen documentation can be generated.
 * Add some documentation comments.
 * Add -O0 compiler flag to the Makefile for debug purposes.