Added #arrange method

`#arrange` method added to `descendants`, `ancestors`,
`self_and_descendants`, `self_and_ancestors` relations
Rails 4.1 support
Added :orphans option to #arrange method

User can choose how to deal with orphans during
nodes arrangement. Possible value for :orphans
options is :discard. By default orphans are treated
as root nodes.
Version bump to 1.3.0
Merge branch 'master' into movement_refactoring

Conflicts:
	.travis.yml
	acts_as_ordered_tree.gemspec
	lib/acts_as_ordered_tree/instance_methods.rb
	spec/spec_helper.rb