Allow launching on a custom Tokio runtime

[jhpratt]

Currently only on a Tokio runtime for reasons described in #1068. In the future, it would naturally be ideal to allow launching on any runtime; such a change would necessitate larger code changes to provide for a common API.

Resolves #1068.

[jebrosen]

launch_on should take &Runtime or possibly TaskExecutor; moving Runtime by value probably precludes most of the useful reasons to do this.

block_on shouldn't be called inside launch_on, for a similar reason. I suspect launch_on will need to return some kind of future or spawn handle.

Ideally we could have a method that took only self and returned a future that could be spawned by the user on any tokio runtime, but we currently need the executor "ahead of time" in order to pass it to hyper.

[jhpratt]

Ah, I originally assumed this was just to be able to customize the runtime, not the ability to launch on a runtime that may already exist and does other things. That's certainly a different situation.

I've opted to accept a &Runtime over an &Executor, as that will be helpful when #1069 is implemented.

[jebrosen]

I would like to merge after these minor adjustments so that #1079 is combined with these changes.

I think both functions will change further, but this is a great starting point to play with the functionality.

[jebrosen]

Merged as 70ca157.