BF,DOC: add import of base.constraints to suite + minor typo
ENH: docstrings for constraints' __init__ and use semantic *constraints,*values instead of pure *args
RF/ENH: constraints -- use __or__ and __and__ for concise semantics, improved testing, EnsureValue -> Constraint
ENH: consistent (more TODO) use of assert_raises + check for Range to not be type dependent
BF: constraint short_description is now mandatory

after @yarikoptic make base class abstract