fix ambiguity warnings

Warning: New definition
    read(Writer,Any...) at /home/fhs/.julia/Zlib/src/Zlib.jl:187
is ambiguous with
    read(IO,Type{Complex{T<:Real}}) at complex.jl:84.
Make sure
    read(Writer,Type{Complex{T<:Real}})
is defined first.
...
etc.
Merge pull request #7 from fhs/ambiguity

fix ambiguity warnings