Added skills system

So the implementation of the skill training system is now complete.  All skill books are currently available for purchase in a country's capital port, and are treated like any other item until you use them.  When you click Add to Library it removes the skill book from your city storage and adds the skill to your character's skill list at level 0.  You can then select it for skill training. 

All skill requirements for all items that are currently in the game have been implemented as far as listing those requirements in code, and showing all items you don't have skills yet for as the item in a slight red color. 

Next in the skills system is implementing restrictions on actually using items if you don't have the skills to do so.  After that, some skills provide bonuses such as faster ship turning, faster water bailing, etc, and those will all be implemented.  The last part (which I may actually work on first), is right now the skill requirements aren't actually displayed in game beyond you have them fulfilled or not, but not what you need for each item.  I'm going to implement the Broadopedia feature which will have detailed information about every item, every feature, every game concept that is in Broadside.