Basic wrapper library for QuickBooks interop

I've just dropped out QuickBooks support for a secret project, so I'm dumping the interop support here.

If somebody is interested in using this, I'm happy to accept help cleaning it up and getting it on NuGet ;-)

Note that the version of the QuickBooks SDK that I developed this against is X86 only, so the project has to be built on X86 (ie, Any CPU won't cut it).