Added code folding for #region/#endregion blocks.

This adds an extra folding block for ``#region`` and ``#endregion``, per how MonoDevelop supports them for C#.  It supports nested blocks, and has proper folding for blocks inside regions.  The basic tests should be implemented (and should pass), including for the ``_addFolds`` method.  For example:

![regionfolding](https://f.cloud.github.com/assets/4183785/1287208/f9d158b0-2fee-11e3-9976-eea903d0874a.png)