Other proxy code cleanup tasks

[Eilon]

These tasks were originally noted in #43:

• Consider renaming the proxy prefix to _proxy. Usually an underscore is used to differentiate 'built-in' features, which this would be.
• Replace new GetKeyValuePairs() method with a built-in .NET API for parsing query string values
• Consider changing the new JS method SendInvokeMessageToDotNetAsync to be another message type (we already have message type 0 (raw) and 1 (invoke method). It could then internally uses the new proxy mechanism, but perhaps not get mixed into the 'first-class' proxy feature? (This could make the actual proxy code a bit cleaner maybe?)

Note: #51 is specifically to address possible hangs due to sync-over-async usage of .Wait().