Changed the spell formulas for better balance.

Some of the spells are out of balance as seen in issue HabitRPG/habitrpg#3542. I decided to play around with these, and here's what I came up with.  Most likely we'll need people to play around with it and figure out if we need to tweak the numbers a bit more, but here's a first whack at it.

What I changed:
Wizard:

	Burst of Flames:
		value: Slightly more effective for low level users and slightly less effective for 
			mid-high level users so its more in line with Brutal Smash and Searing Brightness.

		exp: Changed the calculation for code uniformity.  May be slightly less effective.

		boss damage: Reduced the damage from BoF, more so for the high level users.
			This spell in conjunction with a large mana pool was too powerful

Warrior:

	Brutal Smash:
		boss damage: Increased the damage from Brutal Smash, especially for low-mid level users.
			Gave it diminishing returns, so the damage wouldn't get out of hand.

Rogue:

	Pickpocket:
		code cleanup, but otherwise unchanged

	Backstab:
		exp: Gave it diminishing returns.  Now it is in line with Burst of Flames.

		gp:  Gave it diminishing returns.  The gp gained is less than Pickpocket, since
			Pickpocket only awards gp, while Backstab awards gp, exp, and increases the task's value.

Healer:

	Searing Brightness:
		value: Slightly increased effectiveness

Here are Some charts for a better visual of how its changed.  I did a couple 3d charts since the formula is based on the user's stat as well as the task's value. Keep in mind the scale is different in the 3D charts, so don't just look at the color.

![image](https://cloud.githubusercontent.com/assets/5182425/4565384/71f3c652-4f1f-11e4-9ca0-cc8a6016fb25.png)
![bofold](https://cloud.githubusercontent.com/assets/5182425/4565434/baae2f5e-4f1f-11e4-805b-76af54def2c5.png)
![bofnew](https://cloud.githubusercontent.com/assets/5182425/4565436/bf99396e-4f1f-11e4-8832-1d9414fde4e5.png)