Changed backing data structure

Data was kept in Properties objects, now in a Map<String, Object>. I'm
honestly surprised it was this easy but all unit tests run properly.
Continued implementing YAML

Doc is now outdated, unit tests don't run, never tested if it works... Committing to create backup point.
Working YAML implementation
Fixed a bunch of unit tests

Still some to go...
Fixed a bunch of compiler warnings
Clarified TODO message for 'containsKey'