PyO3 0.3.0

Changes

• Upgraded to syn 0.14 which means much better error messages 🎉
• 128 bit integer support by kngwyu (#137)
• proc_macro has been stabilized on nightly (rust-lang/rust#52081). This means that we can remove the proc_macro feature, but now we need the use_extern_macros from the 2018 edition instead.
• All proc macro are now prefixed with py and live in the prelude. This means you can use #[pyclass], #[pymethods], #[pyproto], #[pyfunction] and #[pymodinit] directly, at least after a use pyo3::prelude::*. They were also moved into a module called proc_macro. You shouldn't use #[pyo3::proc_macro::pyclass] or other longer paths in attributes because proc_macro_path_invoc isn't going to be stabilized soon.
• Renamed the base option in the pyclass macro to extends.
• #[pymodinit] uses the function name as module name, unless the name is overrriden with #[pymodinit(name)]
• The guide is now properly versioned.
• A few internal macros became part of the public api (#155, #186)
• Always clone in getters. This allows using the get-annotation on all Clone-Types