Kinematic Equations- Fixes and Improvements

http://sandcastle.khanacademy.org/media/castles/orimilstein:patch-7/exercises/kinematic_equations.html

Changes:

When OMITTED = v_f or v_i, and UNKNOWN = t, modified solutions to accept only positive time. hints cancel the wrong answer and explain why.

When OMITTED = t and UNKNOWN = v_i or v_f, there are cases when two answers with one positive velocity and one negative is not logical/physically possible. Modified solutions accordingly, and hints cancel the wrong answer and explain why.

Slight changes to wording in hints.

Corrected one case when hint had a wrong unit.

Note: 
1. please excuse me if the formatting is not good, I'm new to programming.
2. I tried to write/rewrite hints to the best of my ability. However, english is not my first language.

Corrections, suggestions and criticism are welcome!