Implement CustomEvent (#2173)

[Manishearth]

This was previously PR'd on #2218

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/1510

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[jdm]

Needs a rebase, and then it's mergeable.

[jdm]

13:57:43     INFO -  /builds/slave/mac/build/src/components/script/dom/customevent.rs:45:73: 45:74 error: obsolete syntax: `~` notation for owned pointer allocation
13:57:43     INFO -  /builds/slave/mac/build/src/components/script/dom/customevent.rs:45         reflect_dom_object(~CustomEvent::new_inherited(CustomEventTypeId),
13:57:43     INFO -                                                                                                                                              ^
13:57:43     INFO -  note: use the `box` operator instead of `~`
13:57:43     INFO -  error: aborting due to previous error