Red- Test added to start the feature of print the grid
Green- Added the minimum code to make the test pass
Refactor- Now the toString method in CellsGrid uses the toStringMethod in Cell to build the resulting string
Red- added a test to verify the printable representation of the grid with live cells
Green- now in toString we added a if to fix the "X" if the cell a live cell
Refactor- using polymorphism to print the correct representation of the cell, and a test was deleted because of its useless
Red- Bug found- Testing with well known patterns the blinkerPattern (http://en.wikipedia.org/wiki/Conway%27s_Game_of_Life) does not passes
Green- Bug fix- the problem was that i was returning always the same grid but modified, now to solve de bug i have created a new grid that represents the new generation
Green- Some tests added for known patterns