Moves khmer exception classes to their own file and defines new base class for all
exceptions.
All exceptions defined in khmer are derived class from khmer::khmer_exception,
including khmer::khmer_file_exception.
fixed issue #86
Added `import traceback` to test utils script
Added test for normalize_by_median.py for false_positive rate checking
Added my changes to the Changelog
Move all exception classes definitions in khmer.hh to khmer_exception.hh.
Edited Changelog for format, changed assert statement in test to remove
unneeded output (and make it PEP8 compliant)
Merge pull request #559 from timiat/fix/402

Fix/402
Change all throw std::exceptions to throw khmer::khmer_exception, so all
exceptions thrown by khmer functions are an instance (or derived from) the
latter.
Reduced memory requirements in test_lump.py
Reduced memory requirements for test_graph.py
Reduced memory requirements for test_hashbits.py
add a test for issue #86 in tests/test_read_parsers.py
Add description of changes to merge branch fix/508.
Fixed pep8 error in test_graph
Updated Changelog
cleaning up cppcheck violations for issue 506
Fix bad formatting in the Changelog.
Resolved conflic