Given a verb in either hiragana or kanji (Preferred), this API allows you to specify which group (I, II, III) it is, give you the root verb as in dictionary and even conjugate using the tense, continuity and negative intention. And much more.
See the Unit test for how to use and the details of the system. The entire API is written in Swift1.2 and Unit test was written with Xcode6.3. It works with Xcode6.4 but might need to change something in Xcode7.