Exclude non-source directories from rdoc.
Merge pull request #301 from troessner/exclude-non-source-directories-from-rdoc

Exclude non-source directories from rdoc.
Introduce RuboCop and fix offenses

* Add RuboCop configuration
* Fix offenses

This does not fix all offenses. These can be fixed as and when the
relevant code is touched. The following offenses are left:

- Metrics/ClassLength
- Metrics/LineLength
- Metrics/MethodLength
- Metrics/ParameterLists
- Style/Documentation