The "authors" return value should be a list (of author dicts),
but in case of failure it returned an empty string.

This was incorrectly handled in Util.summarize(), but
fortunately with no negative effect:
- iterating over a string returns its characters, but
- iterating over an empty string behaves like
  iterating over an empty dict.

Create a cache dictionary, with file paths as key.
Util.get_authors() is called twice, and there's no need
to call Git twice if we can cache the results.

This has been inadvertently been overwritten by copy/paste

Closes timvink#1

This flag for the git.Repo initializer will search parent directories
for a valid Git repository too.

Sometimes authors commit with different email addresses or
change them over time. This commit changes the key in the authors
dictionary to be commit.author.name to merge such "virtual committers"
(if they have differing user names there's not much we can do about it).

In addition this commit also displays the contribution percentage
(since that has already been calculated), but only if there's more than
one contributor to the page.
Eventually I think this should be made configurable (switch on/off
and a template), but this is a more involved consideration about
the interface that I don't want to "do" without a discussion.