Fix for multiline literals generation in TURTLE output

This fix uses RDFlib for proper encoding of literals (it's better to reuse results of someones hard work)

Here's the difference before and after the patch:

```diff
--- before.ttl	2013-12-05 15:16:00.000000000 +0400
+++ after.ttl	2013-12-05 17:09:07.000000000 +0400
@@ -4026,8 +4026,8 @@
     .
 schema:accessibilityAPI a rdf:Property;
     rdfs:label "Accessibility API"@en;
-    rdfs:comment "Indicates that the resource is compatible with the referenced accessibility API (WebSchemas wiki lists possible values).
-     "@en;
+    rdfs:comment """Indicates that the resource is compatible with the referenced accessibility API (WebSchemas wiki lists possible values).
+     """@en;
     rdfs:domain schema:CreativeWork;
     rdfs:range xsd:string;
     rdfs:isDefinedBy <http://schema.org/CreativeWork>;
@@ -4461,10 +4461,10 @@
     .
 schema:audienceType a rdf:Property;
     rdfs:label "Audience Type"@en;
-    rdfs:comment "The target group associated with a given audience (e.g. veterans, car owners, musicians, etc.)
+    rdfs:comment """The target group associated with a given audience (e.g. veterans, car owners, musicians, etc.)
       domain: Audience
       Range: Text
-    "@en;
+    """@en;
     rdfs:domain schema:Audience;
     rdfs:range xsd:string;
     rdfs:isDefinedBy <http://schema.org/Audience>;
```