Added total duration and shared implementations (#74)

Exposed the total animation duration via the `TotalDuration` enum and
the `Tweenable<T>::total_duration() -> TotalDuration` API.

Added a default implementation for some methods of `Tweenable<T>`,
namely `set_progress()`, `progress()`, and `times_completed()`. These
shared implementations are used by all built-in tweenables.

Issue: #31