[#96] Handle encoding errors on harvest object endpoint

When parsing the harvest object content to see if it is an XML file,
etree.fromstring would fail id there are incorrect unicode errors.
Merge branch 'master' into 96-harvest-object-encoding-errors
Merge branch '96-harvest-object-encoding-errors'
Support for single import commands

We are now able to run `paster harvester import` for a single harvest
object or for a single dataset, providing ids or name.