Fixes consistency issues in tips table & take_over.

Fixes a bug in take_over that results in two entries in the tip table
with the same (tipper, tippee, mtime) but different amount.

Also removes unnecessary zero tips created in take_over when
neither of the users currently tips the tippee.

Further description in issue #1704.
Remove unnecessary zeroing of tips in take_over.
Bring back old conditions while I grok this
Start documenting heavy query inline
Finish folding Python comment into SQL
Factor out unique tips query into a temp table
Move show_table upstream
Move a comment to a better location
Correct a docstring bug
Clarify take_over test suite a tad
Merge pull request #1706 from gittip/issue-1704

Fixes consistency issues in tips table & take_over.