Add preliminary support for context.Context

[sebnow]

Add support for the context concurrency pattern. This enabled control
over timeouts and deadlines when making requests to Facebook.

https://blog.golang.org/context

[huandu]

Thanks for your PR.

I'm looking for a clean way to use Context in this library, e.g. bind a Context to a Session instance. It doesn't look good to modify lots of methods to add Context. Anyway, I haven't decided. We can discuss the design in this issue.

[sebnow]

Yeah, I'm not a fan of adding context everywhere, but it makes sense. At least it's clear where the context is used.

The Context documentation suggests not to store it on structs;

Do not store Contexts inside a struct type; instead, pass a Context explicitly to each function that needs it.

This makes sense for long lived structs, such as Session might be. If a context with a timeout is passed to a Session, then requests made after the timeout will all timeout. A new Session would have to be created for each request to work around this.

Having context-aware and non-context-aware methods clutters up the API, but this was done for backwards compatibility. In the future the context-aware methods could be removed and context could be added to the current ones, removing duplication. You still get a bit of "noise" in the amount of parameters, but that's the way Go has decided to go.

I followed the convention used in the sql package (QueryContext, ExecContext, etc), so the API would be inline with stdlib. Oddly enough the http package doesn't follow this convention.