Add date range for physData + ability to grab specific metric

Added the same range functionality you have for sleep data into the phys data. Also added a metric argument to grab a specific metric of interest. Because there are only 5, doing this individually shouldn't be too much of a burden.

In thinking about how I would personally want to use the API, It seemed that the main purpose would be to grab raw data over date ranges (the thing that Basis doesn't allow) that I could then manipulate using other means--it seemed much easier to use numpy functions on the data after it was already in a nice big array. Some folks may want to throw it into R or play with the data in Excel, etc. I'm not sure what you had in mind, but I thought this might be the most simple format for the API.

Awesome work getting this up. I have certainly benefitted from it.

Cheers