Make get_last_github_commit() return a Unix timestamp
Add get_last_commit_date()
get_last_github_commit(): return author (instead of committer) date
Factor out duplicate code into lemon_check_output()
Issue warning only if the GitHub commit is more recent
Fix outdated version warnings — take dates into account

In order to warn the user about an outdated version, we cannot just
compare the SHA1 hash of the last commit in the local Git repository
with that available on GitHub, as there might be changes we have not
yet pushed. For this reason, we need to take into consideration the
dates of the commits, so that the warning is issued only when there
are more recent changes on GitHub.

This merges branch 'fix-outdated-version-warnings'