Relative imports

When including the the resulting Julia files from a.proto and b.proto inside a Julia module Julia complains that it's unable to locate Module A.  Using a relative import when building the Julia files fixes this issue however may not be the desired behaviour in all cases.
generated Julia code:

```Julia
module B

using ..A

using ProtoBuf
import ProtoBuf.meta

type obj
    description::A.Description
    parameters::Array{String,1}
    obj() = (o=new(); fillunset(o); o)
end #type obj
meta(t::Type{obj}) = meta(t, Symbol[:description], Int[], Dict{Symbol,Any}())

export obj
export meta
end # module B
```