Use sqlalchemy outerjoin for compability reasons

[mkofahl]

Just a suggestion: allow use of older sqlalchemy versions by using outerjoin() instead of join(.., isouter=True). In some environments python modules may have to be used from the distro, so packages may be slightly out-of-date. I recognized the requirements.txt, but for now this fix allows me to keep on running this software.

[justb4]

@mkofahl hard to assess impact of this PR. Code was recently added via PR #187, maybe @cezio can review and give green light? Apart from that, we would like to "pin" requirements.txt, as to have reproducible installs via issue #138.

[cezio]

AFAIR, I used .join(isouter=True) there, because .outerjoin() won't work with sqlite properly. But i didn't check this thoroughly, so if this code works fine with sqlite as well, i'm ok.

[mkofahl]

Tested with sqlite3 3.8.7.1-1+deb8u2 in terms it's not crashing. It's definitly not a very forward-looking PR. So I'm fine not merging it, but maybe it helps someone else having trouble with GHC and an can only join an iterable error.

[justb4]

Closing: has been open for over a year, please open issue if relevant.