+ Significantly improve performance of Source::Buffer (fixes #28).
Correct comments in Source::Buffer#line_for.
Simplify Source::Buffer#source_line.
Add rubocop config.

Exclude autogenerated files so that it won't crash (and report offences
in generated code).
Rubocop is 1.9-only.
Fix 1.9 hash syntax in Gemfile.
Add Ruby 2.0 as a suitable platform for rubocop.