Work on standardizing terminology

Beginning the work of making standard, transparent use of terms for various stats.

* Health, not HP.
* Gold, not GP.
* Equipment, not items, gear, etc. (If we'd prefer "Gear" be standard, I could see it, simply due to common parlance--but I think it's the weaker term. Let me know. Definitely not "items" though, because that's ambiguous with pet-related stuff.)
* "Attack" for weapon experience gain, "Protection" for armor damage reduction.
* Expose the formulae for Strength, Defense, and overall experience boost and damage reduction.

Related PR on habitrpg-shared: https://github.com/HabitRPG/habitrpg-shared/pull/42

(I thought I had an issue ticket out there about this, but couldn't find it!)