Add the ability to self check the database.

Subclasses Postgres to prepare for write-wrapping for #1549
and to add self checks for #1180 and #1705.

Fixes #1180, #1705.
Use global self check.
Add currently known orphans to the self_check.

Also run self_check from the homepage updater thread. This way we should
learn as soon as possible about new orphans by Sentry report.
Skip checking of balances while payday is running.
Check that orphans have no non zero tips.
Update to current data, add more comments.
Merge branch 'master' into self-check

Conflicts:
	configure-aspen.py
	gittip/testing/__init__.py
	tests/test_billing.py
	tests/test_charts_json.py
Merge branch 'master' into self-check

Conflicts:
	configure-aspen.py
	gittip/testing/__init__.py
	tests/test_billing.py
	tests/test_charts_json.py
Merge branch 'self-check' of ssh://github.com/gittip/www.gittip.com into self-check
Special case an existing orphan tip.
Merge pull request #1768 from gittip/self-check

Add the ability to self check the database.