Return matches or None from LogParser iteration

Return matches or None from LogParser iteration

Yielding match-object or string is bad style. Iteration over LogParser now
yields either match-object or None. If None is returned (i.e. parsing line
failed), then the problematic input is saved in LogParser object's attribute
last_unparsed_line (until next matching fails).

To make this work we need to run prepare_result on every single line, so
prepare_result cannot return multiple lines at once. Therefore, made
prepare_result also a generator (which makes more sense, anyway).