Streaming support

[crodas]

I added add new method (getResource()) to MongoGridFSFile that basically returns a file-descriptor (per say in reality it's a stream resource) so the file function (such as fread, fseek, fclose) can be used with files stored on Mongo.

How to use it

$conn = new Mongo
$db   = $conn->someDatabase;
$grid = $db->getGridFs();

$file = $grid->findOne(array('name' => 'foobar.txt'));
$fp   = $file->getResource();

fseek($fp, -1024, SEEK_SET);
echo fread($fp, 1024);
fclose($fp);

Some details

1. The code (gridfs_stream.c) looks very similar to gridfs.c, perhaps I could refactor both later to reuse more code.
2. It is holding in memory only one chunk at a time, it could hold multiple chunks with some changes (i.e, to support read ahead), but I strongly belive that memory is more important than a bit of performance in this particular case.
3. getResource() is only returning the resource, no database operation is performed (chunk loading) until fread is called (also fseek just changes the offset position).
4. I wrote tests for what I considered important, fread, fseek+fread and memory consumption (if something is missing let me know).
5. I think would great to make it less verbosed perhaps findOneFs($query) method that will an alias for findOne($query)->getResource()

[crodas]

Fixed a couple of memory leaks.