NSException+Callsite: Remove obsolete file

NSException extension was no longer used.
Callsite: Add documentation, access control
ExampleMetadata: Add documentation, access control
World: Do not subclass NSObject
World: Add documentation, granular access control

Remove World+Singleton.swift, and instead make World's initializer
private.
Merge pull request #188 from modocache/improve-documentation

Improve documentation and add more granular access control