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/clients/PostHeader.java

@@ -0,0 +1,55 @@
+package org.briarproject.api.clients;
+
+import org.briarproject.api.identity.Author;
+import org.briarproject.api.sync.MessageId;
+
+public abstract class PostHeader {
+
+	private final MessageId id;
+	private final MessageId parentId;
+	private final long timestamp;
+	private final Author author;
+	private final Author.Status authorStatus;
+	private final String contentType;
+	private final boolean read;
+
+	public PostHeader(MessageId id, MessageId parentId, long timestamp,
+			Author author, Author.Status authorStatus, String contentType,
+			boolean read) {
+		this.id = id;
+		this.parentId = parentId;
+		this.timestamp = timestamp;
+		this.author = author;
+		this.authorStatus = authorStatus;
+		this.contentType = contentType;
+		this.read = read;
+	}
+
+	public MessageId getId() {
+		return id;
+	}
+
+	public Author getAuthor() {
+		return author;
+	}
+
+	public Author.Status getAuthorStatus() {
+		return authorStatus;
+	}
+
+	public String getContentType() {
+		return contentType;
+	}
+
+	public long getTimestamp() {
+		return timestamp;
+	}
+
+	public boolean isRead() {
+		return read;
+	}
+
+	public MessageId getParentId() {
+		return parentId;
+	}
+}