Adding ability to specify fields

[iterion]

This is necessary in order to access information like CycleTimeDetails from the API (which I've also added here).

[iterion]

This change also suggests that we may want to consider adding something like:

type StoryOptions struct {
    Filters string
    Fields  string
}

And pass those options along to the appropriate methods. We could also move some of the encoding steps to a method on the StoryOptions struct.

I'd love some input on this.

[tchap]

Thanks, will get to this when I have some spare time...

[tchap]

@iterion Might be better to specify fields as ...string so that the interface actually doesn't change and existing code still compiles. We could do it another way I guess, but that would break the interface, which is better avoided unless there is a strong reason for it...