Escape spaces in SSID during join command

Ah, progress. Newer versions of the wifly firmware require the SSID
string passed to join to have spaces replaced with dollar signs. This
appears to have been true since the 2.36/2.45 firmware release
(2012-09-14), which came out a week after I made my last commit that
escapes spaces for the `set wlan ssid` command.

This may explain why getting a new wifly project working has been a pain
in the butt. If it was an older RN-XV board, the code I had would work
just fine. If it were newer, it would only initially work on a network
whose SSID contains no spaces, because the newer join command requires
spaces to be replaced with dollar signs.