Require RSpec v2.14+
Switches to named subjects since RSpec 2.14 does not allow to use subject.call anymore
Fixes RSpec deprecation warnings by replacing calls to stub with double
Spec refactoring

* Any calls to explicit subjects have been replaced with calls to named subjects
* Removes a ton of new lines to improve readability
Merges feature: RSpec update + Spec refactoring
Fixes context title
Changes property conversion to support any Proc object

Properties now exclusively use objects that support #to_proc for conversion.
Relaxes acceptance checking
Simplifies Property#accepts?
Merges feature: Smarter conversion and acceptance checking
Updates version: v1.3.0 -> 1.4.0
Release v1.4.0