Fixed #547 - Missing "contentURL" property on attachments
Fixed #547 - Missing "contentURL" property on attachments

- ports Attachment.getContentURL() from iOS
- removed unused instance variable from Attachment.java

gotmyjobs/couchbase-lite-java-core,mariosotil/couchbase-lite-java-core,netsense-sas/couchbase-lite-java-core,mariosotil/couchbase-lite-java-core,Spotme/couchbase-lite-java-core,couchbase/couchbase-lite-java-core,mariosotil/couchbase-lite-java-core,4u7/couchbase-lite-java-core