Rationalize API after adding maximum size feature.

. Change name of Group to Spec
	(with the addition of the fglexibility field, this class no longer represents a group)

. Replace overloaded Group/Spec constructors with factory method

. Bugfix for measure() when alignmentMode == ALIGN_BOUNDS

. Bury as much Java API as possible, to minimize restrictions on future API enhancements

	- make all field access in Group package private
	- use factory methods in place of field assignment

Change-Id: I46a5027a013bf7c3110b77108b8fd0427165cd18