Update to v203 events & labels API

Label events are removed in the v203 API. There is now a separate API
call for labelling users, exposed as `Sift\Client::labelUser()`. The
factory method `Sift\Event::labelEvent()` is removed accordingly.

There are also many more built-in event types. These are documented at
https://siftscience.com/docs/references/events-api and implemented as
new factory methods on `Sift\Event`.
Implement scores API call
Update README
Add Travis support
Expose event types as constants
Expose generic static constructor
Update README
More event constants
Encapsulate user ID in label class
Add Payload::equals() for testing purposes
Use late static binding in factory constructors

This allows for more predictable behaviour when invoking factory
constructors in subclasses.
Check more explicitly for empty optional args
Use a more descriptive variable name
Fix broken initialisation of default client
Add constants for $login_status values
Use more meaningful example values in docs
Tweak whitespace
Merge pull request #1 from 99designs/v203

Support API version v203