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).
Hide symbols tab with a configuration
Inform that files not containing ontologies are visible in the ontology files tab
Merge branch 'staging' into augmenting349_facilitating_repository_tabs

Conflicts:
	app/views/files/files.html.haml
Merge pull request #409 from ontohub/correcting394_hiding_symbols_tab

Hide symbols tab with a configuration
Merge pull request #410 from ontohub/augmenting349_facilitating_repository_tabs

Aacilitating repository tabs. Fixes #349.
fix history of directory error
fix encoding problem

UTF-8 is assumed. If a file is displayed, unknown characters are replaced by a UTF-8 question mark
Respecting the standard layout of SVN repositories if used.

closes #418
move breadcrumbs partial to files
fix diff view encoding error