Support multiple endpoints for redundancy

[johnnyshields]

Current Behavior

• Not possible to set product_token and endpoint per message

Enhancement

1. Support multiple API endpoints with random sending pattern.
2. It adds configuration.endpoints and makes endpoint= and alias to it. It accepts either a String or Array (internally the value is an array)
3. Enables setting product_token and endpoint per message
4. Add timeout global setting for HTTP timeout with CM endpoints, default value 10 seconds.
5. Update API endpoint to be https://gw.cmtelecom.com

More background

CM provides separate endpoints w/ separate token for mainland china vs. global, hence my app has the use case to set per message.

[johnnyshields]

@itschn please merge this then cut a release.

[coveralls]

Coverage increased (+0.03%) to 99.712% when pulling 69f1eaf on johnnyshields:multi-endpoint into cc1694a on HitFox:master.

[johnnyshields]

@itschn can you merge this please?