Documenting Messages' apostrophe handling, adding messages.path configuration option.

This is a squashed commit. For people looking back with git-blame, here's
justification for my actions.

----------------------------------------------------------------------------
Documentation clarification

"can be" suggests that MessageFormat is optional, but the example doesn't
show any way to override this.

If this is incorrect, a more complte example should be added beside the
basic usage example.

----------------------------------------------------------------------------

Clarifying missing apostrophes for the uninitiated

Not having a Java background, I had never encountered java.text.MessageFormat,
and had no idea why my apostrophes were disappearing.

----------------------------------------------------------------------------

Adding messages.path configuration option

When loading a messages file, prepend messages.path before attempting to
find the resource.