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/BlogManager.java

@@ -0,0 +1,48 @@
+package org.briarproject.api.blogs;
+
+import org.briarproject.api.FormatException;
+import org.briarproject.api.db.DbException;
+import org.briarproject.api.db.Transaction;
+import org.briarproject.api.identity.LocalAuthor;
+import org.briarproject.api.sync.ClientId;
+import org.briarproject.api.sync.GroupId;
+import org.briarproject.api.sync.MessageId;
+import org.jetbrains.annotations.Nullable;
+
+import java.util.Collection;
+
+public interface BlogManager {
+
+	/** Returns the unique ID of the blog client. */
+	ClientId getClientId();
+
+	/** Creates a new Blog. */
+	Blog addBlog(LocalAuthor localAuthor, String name, String description)
+			throws DbException;
+
+	/** Stores a local blog post. */
+	void addLocalPost(BlogPost p) throws DbException;
+
+	/** Returns the blog with the given ID. */
+	Blog getBlog(GroupId g) throws DbException;
+
+	/** Returns the blog with the given ID. */
+	Blog getBlog(Transaction txn, GroupId g) throws DbException;
+
+	/** Returns all blogs to which the localAuthor created. */
+	Collection<Blog> getBlogs(LocalAuthor localAuthor) throws DbException;
+
+	/** Returns all blogs to which the user subscribes. */
+	Collection<Blog> getBlogs() throws DbException;
+
+	/** Returns the body of the blog post with the given ID. */
+	@Nullable
+	byte[] getPostBody(MessageId m) throws DbException;
+
+	/** Returns the headers of all posts in the given blog. */
+	Collection<BlogPostHeader> getPostHeaders(GroupId g) throws DbException;
+
+	/** Marks a blog post as read or unread. */
+	void setReadFlag(MessageId m, boolean read) throws DbException;
+
+}