added tests and documentation for the span within query

docs/Query/SpanWithin.md

+# Span Within query
+
+> More info about Boosting query is in the [official elasticsearch docs][1]
+
+Returns matches which are enclosed inside another span query.
+
+```JSON
+{
+    "span_within" : {
+        "little" : {
+            "span_term" : { "field1" : "foo" }
+        },
+        "big" : {
+            "span_near" : {
+                "clauses" : [
+                    { "span_term" : { "field1" : "bar" } },
+                    { "span_term" : { "field1" : "baz" } }
+                ],
+                "slop" : 5,
+                "in_order" : true
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$spanTermQuery = new SpanTermQuery('field1', 'foo');
+$spanNearQuery = new SpanNearQuery();
+
+$spanNearQuery->setSlop(5);
+$spanNearQuery->addParameter('in_order', true);
+$spanNearQuery->addQuery(new SpanTermQuery('field1', 'bar'));
+$spanNearQuery->addQuery(new SpanTermQuery('field1', 'baz'));
+
+$spanWithinQuery = new SpanWithinQuery(
+    $spanTermQuery,
+    $spanNearQuery
+);
+
+$search = new Search();
+$search->addQuery($spanWithinQuery);
+
+$queryArray = $search->toArray();
+```
+
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-span-within-query.html

tests/Query/Span/SpanWithinQueryTest.php

+<?php
+
+/*
+ * This file is part of the ONGR package.
+ *
+ * (c) NFQ Technologies UAB <info@nfq.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace ONGR\ElasticsearchDSL\Tests\Query\Span;
+
+use ONGR\ElasticsearchDSL\Query\Span\SpanWithinQuery;
+
+/**
+ * Unit test for SpanWithinQuery.
+ */
+class SpanWithinQueryTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * Tests for toArray().
+     */
+    public function testToArray()
+    {
+        $query = new SpanWithinQuery(
+            $this->getSpanQueryMock('foo'),
+            $this->getSpanQueryMock('bar')
+        );
+        $result = [
+            'span_within' => [
+                'little' => [
+                    'span_term' => ['user' => 'foo'],
+                ],
+                'big' => [
+                    'span_term' => ['user' => 'bar'],
+                ],
+            ],
+        ];
+        $this->assertEquals($result, $query->toArray());
+    }
+
+    /**
+     * @param string $value
+     *
+     * @returns \PHPUnit_Framework_MockObject_MockObject
+     */
+    private function getSpanQueryMock($value)
+    {
+        $mock = $this->getMock('ONGR\ElasticsearchDSL\Query\Span\SpanQueryInterface');
+        $mock
+            ->expects($this->once())
+            ->method('toArray')
+            ->willReturn(['span_term' => ['user' => $value]]);
+        return $mock;
+    }
+}