Basic cleanup, documentation corrections

[novas1r1]

Hey, I made some changes to the code. Most of it is cleaning up, providing static methods and adjusting the documentation. Please take a look and let me know, what you think.

• Updated package to make it compliant with flutter package rules (moving everything to /src and only export files which should be usable from flutter apps)
• Made all calls in Telemetrydecksdk class static - no need to create an instance anymore
• additionalPayload takes a Map<String, dynamic>now instead of a Map<String, String> and values will now be stringified automatically

This is just a suggestion, of course it can be improved :)

[winsmith]

Hi, sorry for the late reply! Thank you SO so much for your PR, I appreciate it a lot. While I was away, my coworker @kkostov made some changes to the SDK that introduced conflicts to your PR. Could you please fix those, then I'll merge right away.

[novas1r1]

@winsmith I resolved the merge conflicts and I will push a release of my app in production now with this branches version. Maybe you can also have a quick check again, if everything is fine :)