Recover the time passed since the last execution of update by invoking timeIntervalSinceNow on our lastTick object.
Accessor: start ticking
Accessor: stop ticking
Added block enumeration
How many colors are available in this game
Some comments
Enumeration list
Computed variable for getting the sprite name
Computed variable for adhere to the Printable interface
Random function for choosing a block color
Fixed issue: fromRaw has been changed in Xcode 6.1
Block class
Comments about the Hashable interface: It allows Block to be stored in Array2D
Block color: it is a constant
Block properties: column, row, and sprite
Shortcut for retrieving the block name
hasValue: required from the hashable interface
description: required from the Printable interface
Initialization of the block
Custom operator- == - when comparing one Block with another. Required from the Hashable interface