Support for XLS Sheet Copy between workbooks

Allows Color Palettes, styles, and images to be copied along with the sheet

This resolves the following issue:
http://npoi.codeplex.com/workitem/6057

I needed this feature at work, because we had a bunch of .XLS reports exported by MS Reporting Services that we wanted to be able to combine into a Excel single document. The color palette merge logic was necessary for this because SSRS optimizes its color palettes, so each document only had the colors being used in it's palette.

Some of this code is based on logic in the linked issue, the Image Copy and Palette Merge code was added, as well as fixing the sheet copy bug in that code that was causing all of the text to be messed up.