Do some streamlining and UI changes
Do some streamlining and UI changes

One of the effects of this is that it is possible to spray files into
an existing directory in S3 more easily. Users should just be careful.

* Ditch common ancestor determination

* Cut down PsqlHelper since only one interface mattered

* As an aside, support environment-variable AWS key setting

Signed-off-by: Daniel Farina <3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@heroku.com>

ajmarks/wal-e,intoximeters/wal-e,RichardKnop/wal-e,ArtemZ/wal-e,tenstartups/wal-e,alex/wal-e,heroku/wal-e,fdr/wal-e,wal-e/wal-e,DataDog/wal-e,x86Labs/wal-e,modulexcite/wal-e,nagual13/wal-e,equa/wal-e,fdr/heroku-wal-e