Improvements and fixes to NSTextList-related code

[fjmilens3]

Fixes for some NSTextList-related issues (#293, #294, #295).

Changes include:

• Changes to isEqual: behavior for NSTextList and NSParagraphStyle.
• Includes a default empty array value for NSParagraphStyle _textLists (similar to _tabStops).
• Fixes NSAttributeString's rangeOfTextList:atIndex: to avoid lockup conditions for adjacent ranges.
• Fixes NSAttributeString's itemNumberInTextList:atIndex: to return the item number for the list.
• Includes test cases for the above.

Notes:

• I only installed GNUStep last week and don't have a full setup for building and working on GNUStep itself yet. I was able to build libs-gui with some opt-outs. If I keep running into problems, is the gnustep-discuss list the best place to ask for help?
• In addition to the test cases included here, I've tested the code by including it in category methods within my existing GNUStep installation. They seem to resolve the problems that led me to report the bugs.
• This is my first PR so if there are any general GNUStep code practices I'm missing, let me know and I'll adjust accordingly.