Add support to measure active time of tasks

[lyz-code]

Nowadays the way to manage the time spent in a task in taskwarrior is using hooks to add annotations to the tasks.

Nevertheless if you start and stop a task several times your annotations get overflowed, furthermore that information already exists in the undo.data file.

That's the reason of this merge request. To accomplish that I needed to:

1. Get the available task attributes to know what were the keys of undo.data
2. Create a parser of undo.data and save it in an object called history
3. Create a method to extract the active time of a task from that history.

That features are accomplished by:

• _set_task_attrs method to obtain the available task attributes
• TaskHistory class to parse the undo.data file of taskwarrior so as to be able to manage the change history of a task.
  • If you have a huge history of tasks, the parsing time of the undo.data is quite high, therefore I created the _load_history_from_source or _load_history_from_cache
• active_time method for the Task class to measure the active time of a task based on it's history

Sorry for the huge PR :( . If you change your mind, I can split up this PR into 3 smaller ones.

@robgolding close the other MR if you feel like it.

[lyz-code]

Closing for inactivity