Fix compilation with g++-6, Closes: #16 and automatically run test suite

[gerddie]

This pull request fixes two issues:

• The patch against src/graph/hash_graph.h fixes compilation with g++-6 (in addition to the already applied changes to the ifstream handling) and really closes #16.
• the patch against test/Makefile.am makes sure that the tests are only build when make check is called, and here the testa are also automatically run. The latter is important for Debian packaging.

Both patches were tested on Debian compiling with g++-5 and g++-6.

Thanks for considering applying these patches.

Best, Gert

[loneknightpy]

@gerddie Looks good to me. Thanks for the fix.