Improve text munging

It is a much cleaner interface to let Tweet.munge_text() access the dict
elements directly. This keeps the knowledge about the internals inside
class Tweet.
Move all the static methods to the same section
Simplify output handling

Return a string from Tweet.as_html() and write it directly into the file
from Week.entry() instead of collecting the whole entry in a list first.
Simplify date handling

Prevent an IndexError if no dates could be parsed.