Make sets available for use
Add specs for #181
Pass all enclosed values in strs to the lexer for tokenising

Previously, a seperate logic block would handle enclosed variables and
anything else would be passed to a new instance of the lexer for
tokenisation. Unfortunately, this lead to variables that had array or hash
references being classified as :NAME tokens and then when running with fix
mode enabled, would not be turned back into variables.
Fix end of string detection in #check_only_variable_string

Adjusted the logic in this check so that it allows for any number of array
or hash references at the end of a variable name, while still correctly
detecting that it is only a single variable.
Merge branch 'fix_stuff' into fix-181
Merge pull request #212 from rodjek/fix-181

Handle variables with array & hash references