improved examples/

add Makefile to make all examples via implicit rules
hide main() behind #if defined set in Makefile rather than comment
correct one identifier in fourth example
improved tests

add Makefile
add main
improved src/

silence g++ -Wall -pedantic by
- commenting out one unused variable and
- aligning initialization
make graph file writing optional

we probably want to add a setter that can provide a filename...
Merge pull request #6 from eddelbuettel/feature/improved-examples

improved examples/
Merge pull request #7 from eddelbuettel/feature/improved-tests

improved tests
Merge pull request #8 from eddelbuettel/feature/improved-src

improved src/
Merge pull request #9 from eddelbuettel/feature/optional-graph-file

make graph file writing optional