Support rounding of rationals

Before this PR (on Julia 0.3):

```
julia> using Compat

julia> round(Int, 1//2)
ERROR: `iround` has no method matching iround(::Type{Int64}, ::Rational{Int64})
 in round at /home/tlycken/.julia/v0.3/Compat/src/Compat.jl:45
```

After this PR:

```
julia> using Compat

julia> round(Int, 1//2)
1
```