Add a flag on AFHTTPRequestSerializer to optionally avoid ever requiring the main thread

I have an implementation of integration specs that invokes an instance of `AFHTTPRequestOperation` synchronously using `waitUntilFinished` so that I can make assertions after running a series of calls, each guaranteed to finish before the next begins.  Pretty common pattern.

But I had to hunt down why things were timing out whenever I POSTed multipart forms.  Any reason why a networking library ever needs to perform anything on the main thread?  Would be much cleaner to just remove the `[[NSThread currentThread] isMainThread]` entirely.  Things seem to run perfectly with no issues when I simply kill this block.