Add .promise() function on pools and connections

[trevorr]

This allows existing callback-based instances to be wrapped with promise-based interfaces. While most users can simply import mysql2/promise to use only promises, using both interfaces side by side is necessary to access advanced features like streaming query results.

See https://github.com/sidorares/node-mysql2#using-promise-wrapper

Fixes #28

[unional]

thanks!