init of v4

yet another refactoring, this time with tests
adds TreeNode - data structure that holds todo items
adds colors.py
add colors tests
add first workflows
init++
init++
fix bug that resulted in not printing empty lines

in convert_item of AbstractPrinter there was if clause the checked `if converted:` instead of `if converted is not None`
rename TreeNode to Node
add test for Todoitem
add script to run tests with python 2 and 3
add run tests to coverage.hs
add doctoring
make todos_treen in Todos _private
from now workflows assume that todoflow is installed on python path
add consistency to import formatting
add setup.py
change set_master_item to as_subtodos_of

set_master_item was breaking inmutability of Todos
init readme
new icons

ugly in other way