Add EmbeddedResource parameter to Json and Xml providers

This was triggered by the issue in Twitter: https://github.com/fsprojects/FSharp.Data.Toolbox/issues/1

The problem is that `FSharp.Data.Toolbox.Twitter.dll` exposes the types generated by the JSON type provider to the caller - so the methods in the Twitter API return types generated by the type provider from sample JSON files (I think this is reasonable - if we had to re-write all the type declarations by hand, then this would be hard work).

The problem is that the samples are in a local folder (`json/foo.json`) and when you build `FSharp.Data.Toolbox.Twitter.dll`, you do not want to always keep them around (because referencing the Twitter API will trigger the type provider and the type provider will be looking for them). Embedding the samples inline also does not work - they are just too long and the F# compiler cannot deal with them.

With this change, we can say `JsonProvider<"sample.json", EmbeddedResource="MyLib,sample.json">`. The type provider will first try to load `MyLib` and find the resource there (so, when you are referencing compiled `FSharp.Data.Toolbox.Twitter.dll`, it will find the resource there). If this fails, it falls back to standard directory search (which handles the case when you are compiling `FSharp.Data.Toolbox.Twitter.dll` itself).

I'm not entirely happy with this solution (feels a bit hacky), but I do not think that there is any other way - aside from making the type providers generative, which would be nice thing to do, but AFAIK not something that'll happen very soon.