Handle nil-block in iterators
Merge pull request #338 from troessner/issue-333-fix-nested-iterators

Handle nil-block in iterators
Add SexpExtensions::KwargNode to support required keyword arguments.
Merge pull request #340 from chastell/required_kwargs_support

Required keyword arguments support
Bump version to 1.5.1.
Handle empty source, with and without comments