Add metadata functionality.

[danhper]

This PR will allow us to pass custom metadata to the webdriver, through the user agent string,
which should be enough to handle the use case described in #71.

For example

:ok = Ecto.Adapters.SQL.Sandbox.checkout(MyRepo, [])
Hound.start_session(metadata: %{parent: self()})

Task.start_link fn ->
  user_agent = conn |> get_resp_header("user-agent") |> List.first
  %{parent: parent} = Hound.Metadata.extract(user_agent)
  :ok = Ecto.Adapters.SQL.Sandbox.allow(MyRepo, parent, self())
end

To make this possible, additional capabilities are now passed through the :driver keys, which is a breaking change.

What do you think?

/cc @HashNuke @josevalim

[HashNuke]

@tuvistavie Looks good. Would be nice to have a wrapper around it though (and call it as support for ecto-sandboxed tests). Users/devs don't need to know that we are using user-agent string to make it work.

[danhper]

@HashNuke Thanks for the feedback!
I was also thinking the same thing, but as we do not depend on Plug and I do not think it is worth depending on it simply for this, I was wondering what to do.

What do you think?

[HashNuke]

@tuvistavie ah right. My bad. Just noticed the get_resp_header.
Merge it when you are ready.

P.S: Feel free to make releases whenever you think it is appropriate. Hence the hex access :)

[josevalim]

This callback should not be required as start_session should monitor the caller and shut it down.

[danhper]

I am going to check this, thank you.

[josevalim]

It looks great, I have added some comments. :)

[danhper]

@HashNuke Thank you! I will make a new release once we conclude the discussion in phoenixframework/phoenix_ecto#41 and I merge this PR 😃

@josevalim Thank you very much for the feedback. I just updated the PR, integrating the format you suggested.

[josevalim]