Update Grid.java

- Changed board dimensions to be static, since I declared the array to be static and doesn't need to be dynamic anyway, same with number of mines

- added array to store locations of mines so that they can be easily revealed at game over, rather than searching through the entire board

- added explode(); to reveal all mines when the game has ended

- updated rankUp() calls to omit tiles that don't exist (i.e. off the edges of the board)

- updated random coordinate generator, as the old formula could generate -1, which would crash the game