2.7.0: Multiple fixes & features.

API:

• reconnection_attempts config option added. It is a number.
• open() method is now available. It opens previously closed connection without need to recreate the whole client.
• ping is now pinging only if nothing was sent between pinging period: no excess pings now! Closes #43
• Multiple reconnection bugs fixed.

Deps: upgraded.

Code:

• internal types update.
• Tests upgraded.
• Cleanups and some cleaner rework.