Alpaca-trade-api

- Document updates to reflect the latest API
- Clarify base URL for API endpoint
- Implement Polygon fundamentals API
- Better error handling

- Polygon entities now sort `df` property after sorting it by index
- Clarify Python >= 3.5 requirement
- Add snippet for polygon REST usage

Polygon is a price API service with REST and NATS streaming. This python SDK integrates unifies the interface.

The clock and calendar API helps algos determine if market is open or not, identifying the market close time of the particular date etc.

This release also fixes Entity class to properly use pandas.Timestamp for time-related fields as described in the document.