add params as a first class member of Request

attempting to make it easier to consume params declared on the route. i.e.

```julia
get(app, "/hello/<id::Int>") do req, res
  id = req.params[:id]
  "Hello $id"
end
```

see https://github.com/JuliaWeb/Meddle.jl/pull/9
```julia
for (k, v) in params
    req.params[symbol(k)] = v
```