Add and Edit Lead

[marcemarc]

This is a bit of a work in progress, but might help someone... I needed to add a new Lead instead of a new Deal, the endpoint isn't as nice for a new Deal... and I had to hack this together to work in a very short space of time, so might not be the best way etc, but would tidy things up if you had feedback!

relates to this issue: https://github.com/DavidRouyer/pipedrive-dotnet/issues/56