Date querying

supersedes #95

would require something like to make the tests pass:
```
diff --git a/src/PHPCR/Util/QOM/Sql2ToQomQueryConverter.php b/src/PHPCR/Util/QOM/Sql2ToQomQueryConverter.php
index dc26ade..b80d90c 100644
--- a/src/PHPCR/Util/QOM/Sql2ToQomQueryConverter.php
+++ b/src/PHPCR/Util/QOM/Sql2ToQomQueryConverter.php
@@ -755,6 +755,13 @@ class Sql2ToQomQueryConverter
             }
             $token = substr($token, 1, -1);
             $token = str_replace('\\'.$quoteString, $quoteString, $token);
+            if (preg_match('/^\d{4}-\d{2}-\d{2}( \d{2}:\d{2}:\d+)?$/', $token)) {
+                if (preg_match('/^\d{4}-\d{2}-\d{2}$/', $token)) {
+                    $token.= ' 00:00:00';
+                }
+                $token = \DateTime::createFromFormat('Y-m-d H:i:s', $token);
+            }
+
         } elseif (is_numeric($token)) {
             $token = strpos($token, '.') === false ? (int) $token : (float) $token;
         } elseif ($token == 'true') {
```

then again maybe its not necessary and we actually need to improve the test setup to be more flexible to deal with:
```

1) PHPCR\Tests\Query\QOM\Sql2ToQomConverterTest::testQueries
Original query = SELECT * FROM [nt:unstructured] AS sel WHERE sel.prop > '2013-04-15'
Failed asserting that two objects are equal.
--- Expected
+++ Actual
@@ @@
         'operand2' => Jackalope\Query\QOM\Literal Object (
-            'literalValue' => DateTime Object (...)
+            'literalValue' => '2013-04-15'
```