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

[brototyp]

This is an implementation of #153, as an alternative of #154 which is much easier.

Example usage:

    private func example() {
        guard let piwikTracker = PiwikTracker.shared else { return }
        let downloadURL = URL(string: "https://builds.piwik.org/piwik.zip")!
        let event = Event(tracker: piwikTracker, action: ["menu", "some page", "downloadfile"], customTrackingParameters: ["download":downloadURL.absoluteString])
        PiwikTracker.shared?.track(event)
    }

Upsides:

• Much easier usage. Create the event directly and the track it. No need to create intermediary events.
• Adds only two new functions. A new initializer for the Event and a new function to track an event directly.

[brototyp]

@minitrue22 Do you have any input to this?

[thorstenstark]

I like this way of setting custom parameters. But I'm currently not sure if this may have effects on the compatibility with obj-c.

[brototyp]

What currently works with objc will still work. We didn't change the interface. I'm not sure if we will be able to pull of objc compatibility for the new features.

[brototyp]

@thorstenstark I just implemented the rest, added example code and wrote the documentation.

[thorstenstark]

I'm fine with this.