Pagination Update

Changelog:

• Added configuration options for pagination - refer to the wiki: Customizing the pagination items
• Fixed a design oversight: #nextPage() and #previousPage() will now only change the page if it exists.
  • They also now return boolean values: true for when the page was changed, false if it was out of range.
• Bug fixes:
  • Fixed a bug where #setButton(Integer, GUIButton) would cause a NullPointerException

Fixed Issues

• #1
• #2