Let init handle multiple initial positions
Add isAlive method to Grid
Implement isAlive and change the way cells are stored in grid
Improve tick implementation, change isDead property to isAlive, remove obsolete test
Rename gol.grid to gol._grid to indicate internal (SMELL: it still needs to be accessed by tests so it can't be made private)

Add pending test for spawning of cells.
Specify test target in package.json to enable 'npm test'

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