Allow the MARC field that the _id comes from to be a subfield

An _id can be sourced from a control field, or from NNN subfield a. This
allows the id parameter provided to be in the form NNNb, where it will
get the ID from subfield b of field NNN.
Adding Robin Sheat to the contributers
Resolving CPAN version conflicts
Fixed POD error
Merge branch 'dev'
Fixing marc_xml bugs (Catmandu::Exporter::MARC) requires class methods
and instance methods