add cancelling to apollo requests

[jamiepinkham]

In the current implementation, both the fetch and perform operations of ApolloClient return a Void type. However, it's important, especially in fetching data, to be able to cancel an outbound request/response if, for instance, the user leaves the screen, or any other number of reasons.

[apollo-cla]

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[martijnwalraven]

Thanks for the pull request, that looks great! Nice work extracting the Cancellable protocol to abstract from URLSessionTask.

There seems to be a queue for Mac builds on Travis CI, but I'll merge it as soon as the tests pass.