Implement background task for fetching RSS feeds

* Implemented in briar-core as a `ScheduledExecutorService`
  that gets started when the app starts
* The briar-api has a `FeedManager` interface
  that the UI can use to register and unregister feeds
* In this first iteration, feeds are fetched via HTTP(S), not Tor

Closes #484

briar-api/src/org/briarproject/api/feed/FeedManager.java

@@ -0,0 +1,24 @@
+package org.briarproject.api.feed;
+
+import org.briarproject.api.db.DbException;
+import org.briarproject.api.sync.ClientId;
+import org.briarproject.api.sync.GroupId;
+
+import java.io.IOException;
+import java.util.List;
+
+public interface FeedManager {
+
+	/** Returns the unique ID of the client. */
+	ClientId getClientId();
+
+	/** Adds a RSS feed. */
+	void addFeed(String url, GroupId g) throws DbException, IOException;
+
+	/** Removes a RSS feed. */
+	void removeFeed(String url) throws DbException;
+
+	/** Gets a list of all added RSS feeds */
+	List<Feed> getFeeds() throws DbException;
+
+}