Updated Skills.java

1. getPercentageToNextLevel was not using getBaseLevel but getLevel, which caused to give wrong percentages.
2. getPercentageToNextLevel was giving the progress of the current level, not the percentage untill the next level. (e.g. returned 25%, but should be 75%)
3. Added getSkillName(int skill) which returns you the skill name.
4. Removed public static final int SUMMONING = 23; cause there is no in 2007.

And added a check to see if given skill is within bounds.

Sorry if I did anything wrong, kinda new to GitHub.