Skill system and quests!

- Added skill system based on skill tree, opened with [C]. Learn skills to increase stats of your character. The deeper you go through skill tree, the more overpowered skills you unlock.

- Added quest system. Find green exclamation marks on top of villagers and talk to them to take quests. Upon completing quest talk again to the villager with yellow question mark to get a reward. Grey question mark means that you've started quest, but haven't completed it yet.

- Added enemies to cave world - Cave spiders. There are 3 colored versions of them, but, actually, they are all same.

- Added one of the core mechanic of the game - lose all money when travel to other worlds, but keep when returning back to village.

- Changed the way how coins are dropped as loot, there are now 3 coin types: bronze, silver and gold coins. Bronze coins are worth 1-5 money, silver - 25, gold - 100.

- Villagers now speak sometimes, saying strange things I've came up with ;)

- Changed menu design, yes, once more.

- Bug fixes.

- Other minor changes.