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;
+}