Use derive debug for custom enum and struct definitions

[whimboo]

This PR will fix issue #110.

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This change is 

[whimboo]

It looks like that I need some more derives.

[whimboo]

So I have a problem to make one single enum derivable from debug:
https://github.com/mozilla/webdriver-rust/blob/master/src/server.rs#L21

Whenever I add the derive line here I get the following error which I'm not able to solve:

error[E0277]: the trait bound `<U as httpapi::WebDriverExtensionRoute>::Command: std::fmt::Debug` is not satisfied
  --> src/server.rs:23:21
   |
23 |     HandleWebDriver(WebDriverMessage<U>, Sender<WebDriverResult<WebDriverResponse>>),
   |                     ^^^^^^^^^^^^^^^^^^^^ `<U as httpapi::WebDriverExtensionRoute>::Command` cannot be formatted using `:?`; if it is defined in your crate, add `#[derive(Debug)]` or manually implement it
   |
   = help: the trait `std::fmt::Debug` is not implemented for `<U as httpapi::WebDriverExtensionRoute>::Command`
   = help: consider adding a `where <U as httpapi::WebDriverExtensionRoute>::Command: std::fmt::Debug` bound
   = note: required because of the requirements on the impl of `std::fmt::Debug` for `command::WebDriverMessage<U>`
   = note: required because of the requirements on the impl of `std::fmt::Debug` for `&command::WebDriverMessage<U>`
   = note: required for the cast to the object type `std::fmt::Debug`

@andreastt or @jgraham has one of you an idea? If not we might want to leave this enum alone for now. For every else enum and struct I got the debug added.

[whimboo]

For details also see the last failures in CI:
https://travis-ci.org/mozilla/webdriver-rust/jobs/262158981

[jgraham]

You need to add the trait bound of Debug to U.

[whimboo]

So WebDriverExtensionRoute is itself a trait:
https://github.com/mozilla/webdriver-rust/blob/master/src/httpapi.rs#L139

I can see that Clone + Send + PartialEq is used here, but adding Debug doesn't work.

Which line of code actually needs this update?

[whimboo]

I talked with @jgraham on IRC and we decided to leave the remaining case alone for now. Reason is that it would require updates on various other places which don't seem to be worthwhile right now.

Can someone please do another review of the latest changes please?