argument types in class definitions

It would be nice to be able to specify argument types for methods when defining a Python class via `@pydef`. That is
```jl
@pydef type Foo
bar(self, x::T) = baz(x)
end
```
would turn into `PyObject(baz(convert(T, x)))` given the raw `x::PyObject`. The default `T` would be `PyAny`, as now.

This is especially useful to *suppress* the auto-conversion, via `bar(self, x::PyObject)`. For example, in the `IO` conversion I noticed that we aren't implementing `readinto` correctly: to read in-place, we will have to get the raw `PyObject` and use `PyBuffer` to wrap an `Array{UInt8}` around it.

cc @cstjean