* Implement scan_python to process flow by external python script

Many authors have participed to this effort:

- @jakesmo   https://github.com/jakesmo
- @lassimus  https://github.com/lassimus
- @olibre    https://github.com/olibre

The objective is to extend tcpflow using python language.

The original work is available on @lassimus' fork:
https://github.com/lassimus/tcpflow/commits/master

@olibre has continued the work, and has deeply refactored the original source code from @jakesmo and @lassimus.
Instead of adding a new option -P, this commit reuses option "-e python" and adds three parameters:

- -S py_path=...
- -S py_module=...
- -S py_function=....

Autotools/Automake files have also been fixed in comparaison of original source code from @jakesmo and @lassimus.
CMake files have been updated.

For the Autotools/Automake side, the project builds fine with and without the package python-devel.
However for CMake build, package python-devel is required
This will be improved in a future pull request about CMake.

The resulted tcpflow executable have been tested in many ways:

- built with and without python-devel installation,
- tested with and without options -a, -e python,
- tested in situations where parameters were inconsistent
- tested with mistakes in parameters
- ...

There are also some TODOs withing the source code assigned to @simsong:

    TODO #1 When the scanner cannot initialize it, should we use sp.info->flags = scanner_info::SCANNER_DISABLED?
    TODO #2 Why PHASE_THREAD_BEFORE_SCAN never called?
    TODO #3 Similar to TODO #1

This new feature will amplify the possibilities of tcpflow output data processing 😃

* Remplace XML tag <scan_python_result> by <tcpflow:result>

For more information, see:
https://github.com/dfxml-working-group/dfxml_schema/issues/24

* Fix XML tag name <tcpflow:result>

* Avoid symbols "<" and ">" in XML value

* Rename XML attribute py_function -> function