Merge pull request #445 from ontohub/logic_specific_parsing_callbacks

Logic specific parsing callbacks
Merge pull request #448 from ontohub/augmenting276_add_descriptions_to_symbols

Add comment and label attributes to entities. Fixes #276.
remove logic-mappings creation from links.
remove user constraint from LogicMapping.
Merge pull request #450 from ontohub/correcting420_logic_mappings_generation

Do not create LogicMappings on creation of Links. Fixes #420.
add database-cleaner to rspec.