Add support for Universal Apps

- [x] change to `Profile259` (same as `Profile78` but minus WCF)
- [x] actually update the `Rx-Linq` dependency
- [x] correct the package paths to match Rx packages
- [x] bump to v2.0.0 because there's a lot of "cheese moving" happening here
- [x] scrub the complaining about PCLs in the `README`