Removed unused code: invert_range for ports
PEP8 fixes

Many fixes according to our style guidelines.
Include PEP8 compliance verification in test suite

From now on, PEP8 violations (slightly adjusted according to our coding
guidelines) count as test fails.
Added CONTRIBUTORS.md file

This file lists all contributors, in a way that is common practice among
open source projects on Github.
Changed policy flag dropdowns to multiselectfields

Mulitple SWID-flags and TPM-attestation-flags can be selected,
saved as list, separated by whitspace (tnc-ba/strongTNC#66)
Removed debug print, added policy type object to avoid magic numbers.

(tnc-ba/strongTNC#66)
Removed blacklist flag from package-view and -model

It is save to remove the column 'blacklist' from the table
'package' in the database. (tnc-ba/strongTNC#62)
Deleting policies is working again