add a symmetric socket to the Node class
handle activation and setting of symmetric socket
send a symmetric ping packet if appropriate when pinging nodes
Update BUILD.md

Not sure about anybody else but I needed glm 0.9.5.2 the compile without errors. 0.9.5.0 (as suggested in the current BUILD.md) failed because the precision template parameter was undefined.
Merge pull request #2566 from birarda/master

add a symmetric ping type to handle open servers with clients on symmetric nats
Merge pull request #2575 from brotchie/master

Remember most recently opened script location.
Merge pull request #2576 from brotchie/patch-1

Update BUILD.md
Hitting enter no longer shows chat window when chat is disabled.

When chat is disabled (QXMPP library not available or XMPP disconnected)
hitting enter would show the chat window, even when the Tools -> Chat
menu item is grayed out!