Add tagging support.

Entries can include a (space-separated) list of tags,
separated from the main entry title by `//`.

Reports include also per-tag totals, if any tags
are found.
Use only amount of hours in area time reporting.
Use ` -- ` as a separator between entry and tags.

Note that it's space + double-dash + space: a single double-dash is
*not* considered a valid separator.
Document new tags feature.
Correct docstring for `TimeWindow.all_entries`.
Remove old code variant, already commented out.
Add tests for the new tagging feature.
tests.py: Fix for compatibility with Python 3.x
Address issues raised by @standagh
Merge pull request #19 from riccardomurri/master

Add tagging support.