Add "Fetchable" interface for classes providing data retrieval

Classes for retrieving data must implement the Fetchable interface
which defines the usual fetch*() functions.

library/Icinga/Data/Fetchable.php

@@ -0,0 +1,48 @@
+<?php
+
+namespace Icinga\Data;
+
+/**
+ * Interface for retrieving data
+ */
+interface Fetchable
+{
+    /**
+     * Retrieve an array containing all rows of the result set
+     *
+     * @return  array
+     */
+    public function fetchAll();
+
+    /**
+     * Fetch the first row of the result set
+     *
+     * @return  mixed
+     */
+    public function fetchRow();
+
+    /**
+     * Fetch a column of all rows of the result set as an array
+     *
+     * @param   int $columnIndex Index of the column to fetch
+     *
+     * @return  array
+     */
+    public function fetchColumn($columnIndex = 0);
+
+    /**
+     * Fetch the first column of the first row of the result set
+     *
+     * @return  string
+     */
+    public function fetchOne();
+
+    /**
+     * Fetch all rows of the result set as an array of key-value pairs
+     *
+     * The first column is the key, the second column is the value.
+     *
+     * @return  array
+     */
+    public function fetchPairs();
+}