Ignore unused parameters if method calls super in nested context

This handles the case where super without arguments is called in a nested
context in the method. Sexp#find_node does not find those cases, so a new method
SexpNode#has_nested_node? is introduced.

Includes some rewording of spec descriptions, and a small refactoring of
SexpNode#look_for.
Merge pull request #246 from troessner/issue-unused-parameter-and-super-the-sequel

Ignore unused parameters if method calls super in nested context