bidirectional Symbol conversion

Currently, `PyObject(s::Symbol)` produces a Python string, and `convert(Symbol, o)` converts a Python string object back into a `Symbol`. However, automatic type conversion does not work, i.e. `convert(PyAny, PyObject(:Foo))` produces `"Foo"` not `:Foo`.

One way to fix this might be to define a new Python string subtype, and instantiate this type from `PyObject(s::Symbol)`. That way, it will be usable as a string in Python but `convert(PyAny, ...)` will be able to detect that it is "really" a symbol and convert it appropriately.