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`
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
tidied ArgumentParser setup

Removed repeat of default parameter values, use `store_true`,
`store_false` for `--delete` and `--no-delete` options respectively
+func: limit harvesting to an arbitrary number of records

Closes #3
docs: updated README file; section order and Roadmap
docs: minor improvements to README consistency
docs: added example of harvesting a set
docs: added example of limited harvesting
removed version .dev tag
docs: removed Roadmap, added CHANGES file

Updated latest version information.
docs: more README refinements
Merge branch 'release/1.1'
bumped version number and relase date