[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. (issue [#140](https://github.com/adelevie/parse-ruby-client/issues/140))
- Improved support for targeting by platform: a device type is specified as [`where`
clause](https://www.parse.com/docs/push_guide#options-platform/REST)
- Improved support for advanced targeting: if a channel is specified it's also possible to use `where` clause in it.
I think the tests were inconsistent with Parse REST documentation. That's why I decided to change them.
Previously a simple request with device type set made the test failed because it checked no `where` clause was created but if you want to specify a device type you have to add it as `where` clause.
Same goes with nil channel.
If you want to send a broadcast message, you have to add a `where` clause, hence a simple request could have a where clause and a channel specification.