Added multiple header styles, command line args

This is my first pull request, so I apologize if I've done anything wrong, but here's a summary of what I've added:

+ Headers can now be of any size, and it will correctly strip off trailing ### (assuming the function to split the lines is correct). This should close [issue #2](https://github.com/dangerdan/MarkdownToOpenXML/issues/2)

+ [issue #1](https://github.com/dangerdan/MarkdownToOpenXML/issues/1) can now be resolved as Setext headers (both `=`'s and `-`'s) can now be used, and without `-`'s being confused with horizontal rule below header

+ Command line demo now accepts command line arguments, to enable easier automation

+ Updated readme to let users know they must have Open XML SDK 2.5 to build

Hopefully this is the first of many pull requests, with many more features.