Make SIRange concrete for all Range types

[mbauman]

This simplifies the definition of SIRange, allowing it to wrap all Range types as first class values. This improves inference on the range functions, for example:

Before

julia> @code_typed(length(1s:0.1s:2s))
1-element Array{Any,1}:
 :($(Expr(:lambda, {:r}, {{},{{:r,SIRange{Float64,0,0,1,0,0,0,0},0}},{}}, :(begin  # /Users/mbauman/.julia/v0.3/SIUnits/src/SIUnits.jl, line 44:
        return length(top(getfield)(r::SIRange{Float64,0,0,1,0,0,0,0},:val)::Range{Float64})
    end))))

After

julia> @code_typed(length(1s:0.1s:2s))
1-element Array{Any,1}:
 :($(Expr(:lambda, {:r}, {{},{{:r,SIRange{FloatRange{Float64},Float64,0,0,1,0,0,0,0},0}},{}}, :(begin  # /Users/mbauman/.julia/v0.3/SIUnits/src/SIUnits.jl, line 40:
        return top(box)(Int64,top(fpsiround)(Int64,top(getfield)(top(getfield)(r::SIRange{FloatRange{Float64},Float64,0,0,1,0,0,0,0},:val)::FloatRange{Float64},:len)::Float64))::Int64
    end::Int64))))

This removes the definition of SIRange1 as it is no longer needed. As it was neither tested or exported, I have not done anything to deprecate it. I have kept the abstract SIRanges, even though it is no longer necessary or exported, to keep the interface stable.

[mbauman]

I didn't notice 0.2's tests didn't pass until just now. Updated and good to go.

[mbauman]

Thanks!