The ForbiddenGlobalVariableVariable sniff was a little overzealous and would throw errors for valid variable variables.

The sniff has now been partially refactored.

The following improvements were made:

• Don't throw errors for non-complex variable variables. Issue ForbiddenGlobalVariableVariable false positive #537.
• Correctly handle variable variables with whitespace or comments within the variable.
• Throw an error for each forbidden variable. Previously only one error would be thrown per global statement.
  • The error is now thrown on the line containing the variable, not the line containing the global keyword.
  • The error message now shows the problem variable encountered.
• Throw a warning for non-bare variables encountered in a global statement.
  • This warning has a separate errorcode allowing for easy exclusion.
  • The warning, like the error, will show the problem variable encountered.

The unit tests have been redone to account for these changes.

Fixes #537

@wimg If needed, the warning can be taken out quite easily. Let me know if you don't want it in the sniff.