DecimalArithmetic panics on division or modulo by zero #123

With gval.DecimalArithmetic(), division or modulo by zero panics with decimal division by 0 instead of returning an error. The panic comes out of Evaluate, so any expression containing / 0 or % 0 crashes an application that evaluates user-supplied expressions in decimal mode.

lang := gval.NewLanguage(gval.Full(), gval.DecimalArithmetic())
lang.Evaluate("1 / 0", nil)        // panic: decimal division by 0
lang.Evaluate("1 % 0", nil)        // panic: decimal division by 0
lang.Evaluate("0 ** (0 - 1)", nil) // panic: decimal division by 0
lang.Evaluate("x / y", map[string]interface{}{"x": 1, "y": 0}) // same panic

The /, % and ** operators in gval.go call shopspring/decimal's Div, Mod and Pow directly, and those panic on a zero divisor (Pow divides by the base when the exponent is -1 or less). The default float arithmetic is fine: 1 / 0 gives +Inf.

Reproduced on v1.2.4 and on master (c622ea2). I have a fix and will open a PR.