Create stats class to handle stat modifiers

[scottbass47]

We need a class that can keep track of a stat's base value as well as a list of modifiers that can either be:

• Additive (i.e. +3 damage)
• Percent Additive: add up all the percentages, and then multiply
• Percent Multiplicative: multiply each percent

Example Gun Damage:

• Base Value: 4
• Additive Modifiers: +3, -1
• Percent Additive: 10%, 50% (for net percent of 60%)
• Percent Multiplicative: 15%, 20% (for net percent of 38%)