This commit consists of
* a proc macro to convert rust functions into python functions (`#[function]`),
* a macro to register a function in a module (`add_function_to_module!`)
* Documenting both the old and the new way in the book
This allows using generated code from crates that do not directly depend on pyo3.
E.g.:
```rust
extern crate my_pyo3_wrapper;
use my_pyo3_wrapper::pyo3;
#[py::modinit(rust2py)]
fn init_mod(py: Python, m: &PyModule) -> PyResult<()> {
// ...
Ok(())
}
