move request pooling into hoodie.js core

[gr2m]

I made withSingleRequest & withPreviousRequestsAborted for request pooling in the hoodie.account pooling.

I think withSingleRequest should be the default behavior of GETs / DELETEs and withPreviousRequestsAborted the default behavior for PUTs / POSTs in hoodie.request.

That means if I call hoodie.request('GET', '/_session') several times before the first request finishes, the subsequent calls don't send another request, but simply get the promise of the first.

[svnlto]

+1

[gr2m]

closing in favor of hoodiehq/hoodie-client#81