Updated createEntity() description

The old one was way too longwinded, and didn't actually even properly
explain what it was doing.
Replaced getAllEntitiesWithComponent() with allEntityIDs()

It makes more sense to me, currently, to simply provide systems with a
list of all the available Entities rather than a list containing Entities
with a single Component type. If there was a better way to search for
multiple Component types and return a list of those, however, I'd include
a function for that as well.
Completed definitions of all Library functions

Haven't tried compiling it yet though, so might not work yet.