Support of asynchronous api calls.

[Dr-ZeeD]

I suppose the title says it all ;)

[tejado]

What do you have exactly in mind? callback to a given function when the response is returned? Otherwise it could be solved with additional code around the API.

[Dr-ZeeD]

At the moment I do not see a way to start asynchronous api calls except for starting multiple processes. It'd be nice if something like grequests could be supported :)

[RyanHope]

I could port the backend to use Twisted very easily. That would support async calls. This was actually one of the first thing that I was going to do.

[tejado]

With the latest commit, pgoapi should be thread-safe