Fix: $push should create the field if absent

'If the field is absent in the document to update,
$push adds the array field with the value as its element.'

http://docs.mongodb.org/manual/reference/operator/update/push/
More support on regex searches

- Fixed the $regex operator
- Added the ability to search inside arrays
Merge pull request #67 from ahirata/fix_push

More support on $push and $regex