Store schema version in database. Dev task #50.

If the schema of the database is incompatible with the schema expected
by the code, the database throws a DbSchemaException. LifecycleManager
indicates the error to BriarService, which uses HomeScreenActivity to
show a notification and quit the app.

briar-api/src/org/briarproject/api/db/DbSchemaException.java

@@ -0,0 +1,10 @@
+package org.briarproject.api.db;
+
+/**
+ * Thrown when the schema of the database differs from the schema expected by
+ * the code.
+ */
+public class DbSchemaException extends DbException {
+
+	private static final long serialVersionUID = -4444069279533651710L;
+}

briar-api/src/org/briarproject/api/lifecycle/LifecycleManager.java

@@ -13,8 +13,11 @@ public interface LifecycleManager {
 	 */
 	public void registerForShutdown(ExecutorService e);
 
-	/**  Starts any registered {@link Service}s. */
-	public void startServices();
+	/**
+	 * Starts any registered {@link Service}s and returns true if all services
+	 * started successfully.
+	 */
+	public boolean startServices();
 
 	/**
 	 * Stops any registered {@link Service}s and shuts down any registered