Return proper Invitation objects in SharingManager

Fixes #470

briar-api/src/org/briarproject/api/sharing/InvitationItem.java

@@ -0,0 +1,32 @@
+package org.briarproject.api.sharing;
+
+import org.briarproject.api.contact.Contact;
+
+import java.util.Collection;
+
+public class InvitationItem {
+
+	private final Shareable shareable;
+	private final boolean subscribed;
+	private final Collection<Contact> newSharers;
+
+	public InvitationItem(Shareable shareable, boolean subscribed,
+			Collection<Contact> newSharers) {
+
+		this.shareable = shareable;
+		this.subscribed = subscribed;
+		this.newSharers = newSharers;
+	}
+
+	public Shareable getShareable() {
+		return shareable;
+	}
+
+	public boolean isSubscribed() {
+		return subscribed;
+	}
+
+	public Collection<Contact> getNewSharers() {
+		return newSharers;
+	}
+}