[Push Notification] Added support for no channels
[Push Notification] Added support for no channels

## Push Notification:
- If there are no channels (default no channel), the push notification
is sent to all target devices.
- Improved support for targeting by platform: a device type is
specified as [where
clause](https://www.parse.com/docs/push_guide#options-platform/REST)

nickuya/parse-ruby-client,esergion/parse-ruby-client,fabiomassimo/parse-ruby-client,nickuya/parse-ruby-client,esergion/parse-ruby-client,adelevie/parse-ruby-client,adelevie/parse-ruby-client,fabiomassimo/parse-ruby-client,kidlab/parse-ruby-client,kidlab/parse-ruby-client