v0.6.1 - Bug fixes, connection longevity improvements

This release:

• Fixes a bug where exceptions caught while connecting were mis-reported and sometimes caused the connection future to not fire. See #261; thanks, @ylgrgyq!
• Sends HTTP/2 PING frames when the connection is idle to prevent connections from closing unexpectedly (see #266).

For a complete list of changes, please see the v0.6.1 milestone.