generate User-Agent string from declarative version numbers

[aoberoi]

Currently this is hardcoded as "OpenTok-DotNet-SDK/2.0.0". This becomes brittle for maintenance. Infact it is already wrong because this is not a release version of 2.0.0, it will in fact eventually become the 2.2.0 release, but at this point it should be something like 2.2.0-alpha or 2.2.0-development so that we can separate this from release builds that users use (it eventually makes its way into analytics).