Integration of Todo.txt into DoManager

[viper3400]

• DoManager should be able to interact with a todo.txt as specified at http://todotxt.com/.

• Therefore existing source code https://github.com/benrhughes/todotxt.net (under BSD licencse) may be partly used.

• Load Due Tasks from Todo.txt #20 Simple proof of concept integration

• Make Path to ToDo.txt Configurable #21 Configurable path to todo.txt

• Useful integration of Todo.txt