Added a number of pandas-based plotting methods

Reads a TagTime logfile, as for example produced by the TagTime app, and
displays it in various ways.

Currently supported are:

    - A pie chart of the total percentage time spent on a task
    - Bar charts showing which tags were active
      - on a specific day of the week
      - on a specific hour of the day
    - A line plot showing which were active on a given date

The plots can be configured from command line, especially:

    - which tags are to be shown (specific)
    - automatically select the top N tags
    - whether to show the 'other' category in the plots as well
    - the hour resolution in the hour of the day plot

REQUIREMENTS

This script relies heavily on the pandas module, which does most of the work.