Change "Iterable" interface to "Traversable"

The interface originally implemented in the UString class documented in
the `README.md` file was an invalid "Iterable" interface.
PHP has no "Iterable" interface, however a "Traversable" interface does
exist (and seems to have been the intended choice.
Removing `abstract` keyword from the class doc

The `abstract` keyword was improperly denoting the UString class as an
abstract class in the class signature in the `README.md`
Merge pull request #16 from Rican7/bugfix/readme-inaccuracies

Change "Iterable" interface to "Traversable"