Feature/register placement

Changes in this pull request

Renames register(event:) to register(placement:).
Renames preloadPaywalls(forEvents:) to preloadPaywalls(forPlacements:).
Renames getPaywall(forEvent:) to getPaywall(forPlacement:).
Renames getPresentationResult(forEvent:) to getPresentationResult(forPlacement:).
Changes a lot of internal references from event to placement.
Renames the TriggerResult eventNotFound case to placementNotFound.
Renames the PresentationResult and PaywallSkippedReason noRuleMatch case to noAudienceMatch.

[x] All unit tests pass.
[ ] All UI tests pass.
[x] Demo project builds and runs.
[x] I added/updated tests or detailed why my change isn't tested.
[x] I added an entry to the CHANGELOG.md for any breaking changes, enhancements, or bug fixes.
[x] I have run swiftlint in the main directory and fixed any issues.
[ ] I have updated the SDK documentation as well as the online docs.
[x] I have reviewed the contributing guide