Add CodeReference Model.
Add CodeReference to OntologyVersion.
get code positions from hets.
add method to create code-reference from a range.
add method to determine code reference path

Since we now generate Codeposition files with
the -o pp.xml option of hets, we will need
to determine the path to that file.
add method code reference creation from xml doc

Provided a Nokogiri xml doc and an ontology
name the corresponding code-range can be extracted
and a CodeReference object can be created.
Integrate CodeReference creation with import.
make ontology import tests green again

add fixture files for codepositions and add the
path to these files to the import call.
Merge branch 'staging' into augmenting326_parse_do_children_fully

Conflicts:
	app/models/ontology_version.rb
make hets spec pass again (use pp.xml path).
Merge branch 'staging' into augmenting326_parse_do_children_fully
adjust all import tests for new import_xml call.
add owl-generations code fixture xml.
allow for codepos-file to be nil when importing.
supply no codepos-file for links.xml based test.
refactor fixture-methods into test_helper.
move coderef-by-range creation out of the model.
refactor coderef methods for consistent naming.
Merge pull request #406 from ontohub/augmenting326_parse_do_children_fully

Parse distributed ontology children fully (with code references), fixes #326.