Adds Triangle area exercise.

Fixed some bugs in my repository, and wanted to submit this pull request again (cleaner). It removed more advanced vocabulary based on Christi's comment. At approximately the 6th grade level.

Common Core Standard: 6.G.1

Sandcastle:
http://sandcastle.khanacademy.org/media/castles/nphershm:area-tri/exercises/triangle_area.html