Added toString support

[ADD] Added toString support to specifications

[ADD] Added toString tests
[CHANGE] Version

[FIX] Added missing methods to interface

v2.1.1

[FIX] Added missing methods to interface

Added andNot and orNot specification with corresponding tests

v2.1.0

Update package.json

Changed access modifier for properties in specification classes to private

[CHANGE] Changed access modifier for specification class properties t…

…o private and therefore altered public api

Initial release

v1.0.0

[ADD] Missing dependency