MarcExtractor, replace magic spec Hash with a MarcExtractor::Spec object.

Simple substitution, first step in refactor
MarcExtractor::Spec, move indicators from array to seperate values
MarcExtractor::Spec, move #includes_subfield_code? into spec
MarcExtractor rename spec_covering_field to specs plural, it returns 0 to n now
MarcExtractor#spec_hash self-inits to an array
MarcExtractor, rearrange and correct inline docs
MarcExtractor::Spec, redundantly include tag in Spec for reference
MarcExtractor doc fix, LDR not yet supported
MarcExtractor docs tweaks
Merge pull request #40 from jrochkind/extractor_refactor

MarcExtractor, replace magic spec Hash with a MarcExtractor::Spec object...