Add Game Center Capabilities

[psturmfels]

This is a huge one. I'd like the game to be fully integrated with Game Center. This includes the following:

• [x] Add leaderboards for all of the different game modes.
• [x] Add relevant achievements.
• [x] Have achievements give users some measurable, spendable currency.
• [x] Design stuff to buy using the above currency. Link the currency amount and the stuff bought to the user's Game Center account, and have them be able to recover their purchases with their account.

Stretch Goals:

• [ ] Add a multiplayer mode using multipeer connectivity. This would involve two different mellows in the same screen, both climbing together. I'm yet unsure what type of interaction the two mellows would have, if any.

[psturmfels]

Made some progress with this in recent commits. The game has a leaderboard for classic mode!

[psturmfels]

This is finished!