Make SIRange concrete for all Range types

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.
Fix missing UnitRange for Julia 0.2
Small nonsi tweak
Merge pull request #30 from tlycken/identities

Multiplicative and addititive identities
Merge pull request #29 from mbauman/rangetype

Make SIRange concrete for all Range types
one(x::SIQuantity{T}) returns T