Restore the previous `spring.profiles.active` logic effectively
reverting commit 37069d91. This ensures that active profiles defined in
a higher `PropertySource` replace those lower down.

In addition, add support for a `+` prefix that can be used to indicate
that a profile should be added to the active set.

For example:

	# application-prod.properties
	spring.profiles.active=+proddb,+prodmq

Fixed gh-308, gh-309