WIP: Added a way of setting customTrackingParameters to an Event and tracking custom Events.

[brototyp]

Implements: #153

• Added a event() function to the PiwikTracker that returns an empty event that has core properties already set, such as siteId, visitor, session and such.
• Added a let customTrackingParameters: [String:String] property to the Event
• Added a convenience initializer to the Event that copies over all properties from another event and overrides some.
• Added a function to the PiwikTracker that tracks a custom Event

See all those things in action over here.

Todo:

• Discuss if this is a good solution
• Add Documentation to the new public functions
• Add an entry in the Changelog
• Add documentation to the Readme

[thorstenstark]

Adding custom parameters sounds like a good idea. But what happens when the custom parameters interfere with some (new) piwik parameters? Should piwik-owned parameters be protected in some kind from being overwritten?

[brototyp]

@thorstenstark That's an interesting question. I think it is fine, that the user is able to override existing properties. But we should say so in the documentation.

[brototyp]

An alternative implementation got merged.