Parallel requests options

Hi All,

During our work with mixpanel_client we are very often need to ask Mixpanel for different stats during the one server side request. It takes so long and user needs to wait till our app will receive f.e 10 synchronous responses from Mixpanel. Our idea is to use the parallel requests aproach in such cases, so we would like to suggest to improve mixpanel_client and add :parallel option as a client config.

I've attached pull requests with optional :parallel config option(false by default) and update README with an example, If you could take a look and play around with this change and let me know what do you think about this direction of improvements.

It works fine for us, We are going to test it deeply and add some benchmark to compare parallel vs non parallel in the free time.

Let me know if you have any questions or sth is unclear.

Regards,
Greg