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.
Move all exception classes definitions in khmer.hh to khmer_exception.hh.
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.
Add description of changes to merge branch fix/508.
Fix bad formatting in the Changelog.
Move all include's at the beginning of the file.
Merge branch 'fix/508' of github.com:iglpdc/khmer into iglpdc-fix/508
fix ChangeLog formatting
Merge branch 'iglpdc-fix/508'