Added .coveragerc
Fixed name of program in usage output
Abstracted away generator printing

Refs tnc-ba/strongTNC#110.
Renamed swidgenerator_argumentparser module

The module `swidgenerator_argumentparser` is now simply called
`argparser`. The context is obvious at import time due to the relative
import.
Removed double newline special case
Converted OutputGenerator methods to module level functions
Optimized full SWID generation

Previously, when obtaining the list of all files per package, it was
all done in the beginning, before generating all the XML tags. Now the
files per package are fetched just before creating the XML tag, thus
speeding up the time until XML tags are outputted.