Include software and sofware_version in v3bw files

Created by: juga0

Fixes #96 (closed). I thought it would be better to move all that has to do with the generation of the v3bw files to lib/v3bwfile module and leave generate.py basically with only the main. This way we can start to separate better what is obtaining measurements and what is generating the files for the bwauths, as suggested by arm in IRC. Next step would be to test generate complete file including header. For that i would remove what has to do with header in core/test_generate.py (for instance, https://github.com/pastly/simple-bw-scanner/blob/master/tests/core/test_generate.py#L101) and use the header class. It can be done in that file, or move V3BWLine to lib/test_v3bwfile.py. @pastly what do you think?