Standalone DLL for interacting with DOT

This pull request adds a Windows project libdot which will builds a single standalone x86 or x64 DLL (~800KB) with no dependencies and a simple API to access the DOT layout algorithm.

I use an ifdef across a number of source files to get everything to link correctly, but it shouldn't impact the normal build. The project files are from Visual Studio 2012. I also added a file entries to .gitignore to keep out some of the Windows build files.