Interact with Taskwarrior following the 3rd party guidelines

This is a follow up to https://github.com/ralphbean/bugwarrior/issues/64 which discusses following Taskwarrior's [3rd party application guidelines](http://taskwarrior.org/projects/taskwarrior/wiki/3rd-Party_Application_Guidelines).

In your `~/.taskrc` file, add this line:

`taskw.experimental=true`

Then any usage of `taskw` will use subprocess to interact with Taskwarrior's database, by using the `task add`, `task do`, `task export`, and `task modify` commands.

Seems to work fine in my testing.

In `utils.py`, I made a copy of `encode_task` but the only thing I need to change was to remove the brackets around the task description, so this might be revisited.