Don't join repated subfields if a single subfield is specified
test for SolrMarc-style 020aa to signal to join even though one subfield
Refactor and document repeated-subfield syntax
Refactor the refactor to make the repeated subfeild logic more explicit
Remove spurious if/then, and vastly improve repeated-field docs
Tweak MarcExtractor single subfield special behavior docs.

Put in MarcExtractor with (IMO) more clear text/examples,
README just has summary pointing to MarcExtractor for more info.
Merge pull request #39 from jrochkind/repeated_subfields

Don't join repated subfields if a single subfield is specified