Introduced visitor for output generation

Added visitor to print output, no caching in list anymore

Moved document_separator to visitor

Cleaned up comments

Removed debug print

Style fixes + print function

- Imported print function
- Removed unnecessary parens from from if
- Removed unused import

Added file encoding header
Changed identation of pretty output from tab to two spaces

Fixes https://github.com/tnc-ba/swidGenerator/issues/14
Removed extra newline from software-id mode

Fixes https://github.com/tnc-ba/swidGenerator/issues/15
Removed visitor pattern and introduced generators

- Introducing generators allows lazy generation of software-ids and SWID
tags
- Removed print visitor. Output printing is now done with a print function which takes the generator

Added docstring to create_swid_tags method

Changed to use relative imports

Removed obvious keyword from create_swid_tags and create_software_ids

Moved closing parenthesis to previous line

Removed unneeded parentheses

Changed docstring for create_swid_tags

Removed unused parameter
Switched license to MIT

- Updated information in setup.py
- Added LICENSE.txt
- Added license information to `swid_generator/main.py` and `README.rst`

Fixes tnc-ba/strongTNC#97.
Added exit code 1 if no package is found

fixes https://github.com/tnc-ba/strongTNC/issues/104