Added a session ID, that actually tracks the Realm constructor in our RPC server's JS context. This conveniently let us remove the special casing for defaultPath as well as letting us call the Realm constructor with JSObjectCallAsConstructor.
Fixes#68
Since notifications are called synchronously after a write, we fake it by calling them manually for now. The future plan will be more involved, so some of that is stubbed out.
The deleteTestFiles() function is now added to the Realm object, so that it's easy for the RPC module to handle this as well. We can only do this in testing environments in the future.
An exception was thrown when parsing the JSON when only a string was passed. This is better and no longer uses the Objective-C API for converting the dictionary to a JSValue.