Use rb_define_*method instead of DEFMETHOD so RDoc can parse these methods.

before:
![screen shot 2014-07-27 at 12 52 59 pm](https://cloud.githubusercontent.com/assets/277819/3714961/aed231bc-15c7-11e4-82f4-dca59c47bd75.png)

after:
![screen shot 2014-07-27 at 12 53 05 pm](https://cloud.githubusercontent.com/assets/277819/3714962/b4265576-15c7-11e4-8e45-aacd0d36cda1.png)