New Main Menu Button: Continue Game

I've:
1. Created this button.
2. Added English strings for it to the English language file.
3. Added Polish strings for it to the Polish language file (Provided by my Polish friend).
4. Added Russian strings for it to the Russian language file (Provided by my Russian friend).
![continue_button](https://f.cloud.github.com/assets/6353342/2303167/dbab7582-a1a4-11e3-8f26-2e60868db481.png)