add soil conductivity & soil pH about b-parasite

[rbaron]

Hey @MarkAtwood,

I really like the idea, and we had a similar request in the past (#38). I think on the technical side it wouldn't be to difficult, but it makes me wonder about the changes in the fabrication process/cost and battery life (e.g.: can we still use a 3V CR2032 for years?). Right now we have a basic capacitive design that's relatively easy to build, not too expensive and simple to maintain. I am very open to suggestions and I would love to be proven wrong, but if the design and cost are too different at the scale of a few units, I think it warrants its own, separate project.