Blog Client with Factory and Validator

This implements a simple initial blog client that covers the basic blog
actions, but no deletion/removal of blogs, yet.

Classes for Blogs and Blog Post Headers have been introduced along with
the associated factories.

A `BlogPostValidator` has been added that validates incoming blog posts.

Closes #402
Closes #404

briar-api/src/org/briarproject/api/blogs/Blog.java

@@ -0,0 +1,32 @@
+package org.briarproject.api.blogs;
+
+import org.briarproject.api.forum.Forum;
+import org.briarproject.api.identity.Author;
+import org.briarproject.api.sync.Group;
+import org.jetbrains.annotations.NotNull;
+
+public class Blog extends Forum {
+
+	@NotNull
+	private final String description;
+	@NotNull
+	private final Author author;
+
+	public Blog(@NotNull Group group, @NotNull String name,
+			@NotNull String description, @NotNull Author author) {
+		super(group, name, null);
+
+		this.description = description;
+		this.author = author;
+	}
+
+	@NotNull
+	public String getDescription() {
+		return description;
+	}
+
+	@NotNull
+	public Author getAuthor() {
+		return author;
+	}
+}