Implemented Grid, Square, Entity, Unit, Player

Got all the basics working. Wondering about how we’re going to
implement two players colliding and dying though.
Clean up

Implemented suggested changes + Box class. Testing still to come, will
finish it sometime tomorrow.
Ready for Merge (I hope)

Added JUnit tests for Grid, bit of touch up in Box, Grid, Square classes
Fixed Grid i/j issue

It’s a bit awkward, but because printing has to be done row by row,
internally the x coordinate is the last one. This however is made
completely transparent to clients of the class, so using Grid you can
assume regular (x, y) coordinates. Square is a little redesigned for
the fact it can only have one impassable entity. Also TestSquare added
as it helped in debugging.
Merge pull request #19 from oharaandrew314/#5-Grid-and-Square-Entities

Implemented Grid, Square, Entity, Unit, Player