Prefer #to_h over #to_hash

Ruby 2.0 adds #to_h as the standard "serialize this into a Hash" method in core/stdlib, so prefer this over #to_hash
Document #to_h as the preferred method
Merge branch 'to_h-ftw' of git://github.com/foca/hashifiable into foca-to-h-ftw

Conflicts:
	lib/hashifiable.rb