Extending the library

[elazarl]

Hi,

I have a couple things missing, let me know if you'll accept a pull request:

1. In the As{String,Byte...) family of functions, I'm missing AsJson(&jsonStruct),
2. In the As{String,Byte...} family of functions, I'm missing the response status code. In order not to break backwards compatibility, we can add AsStringStatus() (sstring, status int, err error).
3. Accept url.Values for url parameters. Say, add a ParamForm function. To be more consistent with the http pacakge

If that's OK, I'll submit a pull request.

Thanks,
Elazar