skipping deleted records
added capability to selectively harvest by set

Closes #2
compatibility with python < 2.7

`from __future__ import with_statement`
bugfix: harvesting from servers without sets

Fixes bug introduced by 6d85d4035b6cd1362af288bda33579050285424a
whereby servers that don't support sets would always raise
`NoSetHierarchyError`
fixed indentation
respect deletions unless over-ridden

Respect server's request regarding deleted records unless
over-ridden by the command line option `--no-delete`
+func: sensibly handle deletions

Merge branch 'feature/deletions' into develop

Closes #1