^(x::Number,y::Rational) should promote y to the correct type

`rational.jl` contains the rule:

    ^(x::Number, y::Rational) = x^(y.num/y.den)

which always promotes `y` to `Float64` (for `Rational{Int}`) regardless of the type of `x`.  This is too *little* precision if `x` is a `BigFloat` (in which case the exponent should be computed in `BigFloat` precision), and is too *much* precision if `x` is `Float32` (in which case we want a `Float32` result).

The attached patch corrects this by adding two extra definitions (and a test case):

    +^{T<:FloatingPoint}(x::T, y::Rational) = x^(convert(T,y.num)/y.den)
    +^{T<:FloatingPoint}(x::Complex{T}, y::Rational) = x^(convert(T,y.num)/y.den)