get_str/parse_str methods for C native types

[mario-tux]

In a similar way as in report #16, I've found that the core base doesn't provide get_str/parse_str methods for native types. This is a pity as the containers provide them if the elements do.
I'm trying to use such methods to get a quick non-binary serialization of the containers. This would allow to send them as network messages.

[P-p-H-d]

I have implemented the method _get_str for C native types in master.
However, it has raised an issue when running the test suite as described in #23 . If you have some time, I would like to have your opinion on it as it will generate an API breakage.

EDIT: I have implemented the method _parse_str for C native types in master.

[mario-tux]

Thank you!