Update code that checked for :nil nodes

The :nil nodes were removed in Parser 2.0.0.beta6.
nil, false and true are also literals to consider
Use node.loc.something.is? instead of node.loc.something.source ==
Specify more precise highlight points for some offences
Allow rdoc syntax in leading comment space check

rdoc has syntax notations like #--, #++, #:nodoc:, etc.

This fixes #277.
Don't count elsif nodes as block nesting (fixes #252)
Add an explanation about multiple formatters to --help text
A couple of improvements to the AccessControl cop

* added class level documentation
* added handling of `public` modifier
* improved cop messages
Improve the Alias cop a bit

* added class-level doc
* added special handling for aliasing global variables
Add basic documentation for a couple of classes (#208)
Add basic documentation for a couple of classes (#208)
Add basic documentation for a couple of classes (#208)
Add basic documentation for a couple of classes (#208)
Add basic documentation for a couple of classes (#208)
Add basic documentation for a couple of classes (#208)
Enable the Documentation cop

Now that the related Parser bug is fixed we can start using the
Documentation cop.

Hopefully the failing build will motivate the team to document the
offending classes :-)
Fix an error in BlockNesting
Don't run Documentation cop on test code
Refine Documentation cop's offence message
Fix a broken spec