Add Dict conversion method.

I was somewhat miffed to find the following:


```
julia> convert(Dict{String, String}, ["a" => "a"])
ERROR: no method convert(Type{Dict{String,String}}, Dict{ASCIIString,ASCIIString})
 in convert at base.jl:13
```

So I added this method to fix it. The type parameters are admittedly somewhat silly but it seems to work fine.