Redesign ical4android API

[rfc2822]

Objects should be immutable or at least less stateful; API should be well-designed, consistent and well-tested.

Open questions and ideas:

• How much decoupling from DAVx⁵ is really required or desired?
• How much extendability is required/desired and how should it be implemented?
• Calls that can't work shouldn't be possible; get ideas from other persistence frameworks
• Complete redesign (2.0) or improve incrementally?
• Test-driven or at least heavily tested from the beginning

See also bitfireAT/vcard4android#2

[rfc2822]

Closing because at the moment there's no time for that. Will be reopened.