Implement several HTMLizer improvements

This branch introduces several changes in the HTMLizer code, making the text of the tasks more relevant, and visually hiding some implementation details of the todo.txt format which are perfectly fine in the command line, but a little distracting in a desktop UI.

In a nutshell, with this changes the tasks are always aligned, the priority is shown in a cleaner way, creation and completion dates are diminished and displayed at the end.

This branch was originated from the ideas discussed in the Issue #31.

Also, several doctests were adapted to conform to the new display format.