Define .tick() method responsible for making the fallingPiece fall

* Increases fallingPosition by 1
Fix test. Falling happens on the lines, not on the columns.
Define .movePieceLeft() to move current falling -1 square on the X axis (columns)
Rename .place() parameters to avoid confusion

* Rename x to row
* Rename y to col
Add verbs to Board methods

* Rename fallingPosition() to getFallingPosition
* Rename fallingPiece() to getFallingPiece
Prevent fallingPiece from being set out of boundaries

* Add Board#setFallingPosition
* Move piece to left only if its not at X axis boundary
Refactor tests

* Piece is being moved to left on beforeEach, no need to move it again
* No need for other board to test if piece does not go out of boundaries
Move falling piece one sqaure to the right
Prevent .getFullBodyState() from being destructive

* Add isEmpty() method to return if body contains only zeros
* Create Board#getFullBodyDeepClone() to clone each one of the Array's items as .slice() makes only a soft clone and the body attribute is an array of arrays
* Add a test to verify it
Prevent .movePieceRight from setting a piece out o the board