Faraday integration default instrumentation

[marcotc]

Fixes #786

This PR adds a default Faraday middleware handler to allow for ddtrace users to be able to instrument it out-of-the-box, by simply enabling it:

Datadog.configure do |c|
  c.use :faraday
end

Currently we also require users to explicitly configure our middleware on every Faraday connection:

connection = Faraday.new('https://example.com') do |builder|
  builder.use(:ddtrace)
  # ...
end

This per-connection configuration won't be necessary anymore, unless custom configuration settings are desired for that connection.

[delner]

Nice one! Looks good overall; users should be able to use the default instrumentation or connection-specific configuration after this.

One minor issue affecting the tests, but otherwise seems fine to merge to me.

[delner]

👍