Adding timeouts to all remote requests

[soar]

Today I faced problem, when thread became unaccessible because of long answer from Docker Hub registry. I think, all requests to remote APIs should have reasonable timeouts. For me - it's about 10 seconds. If API is not responding - I prefer to get exception and not wait for a lifetime. But Python Requests library has no timeout by default.

So I've added optional parameter, which can be used like:

client = DockerRegistryClient('https://index.docker.io', api_version=1, api_timeout=10)

where api_timeout could be float or tuple according to requests docs.