playbook default location

[thorinaboenke]

This PR solves issue #64

• check if the playbook-file is actually a valid filepath and use that

• if not, check if the playbook is inside the current directory

• if not, check if the playbook is in /etc/attackmate/playbooks

• adapt docs

• add unit tests for parse_playbook

[whotwagner]

I suggest:

1. check if the playbook-file is actually a valid filepath and use that
2. if not, check if the playbook is inside the current directory
3. check if the playbook is in /etc/attackmate/playbooks

[thorinaboenke]

implemented as Hoti suggested, lets not overcomplicate it :)