Parse and provide next page params for device listing, add device count API method

All the device listing API calls take an optional start and limit parameter and will return a next_page field in the response if there is more than one page of data.  I added options ={} to all the device listing methods and parse the next_page out of the response (if present) and inject the start/limit parameters into the root of the response hash.  I also found that the device token count method was missing and added that.

- parse the device_listing responses and inject the start/limit from the next_page URL into the response hash
- Add a missing API Method: /api/device_tokens/count/
- Readme update