Working code - Enables Client Connect/Disconnect detection with AP mode on CC3200 and CC3100.
Added clearing of in_use to beginNetwork() init of AP mode device array
Added deviceIpAddress() and deviceMacAddress() for walking the list of attached devices.  MAC output generates a string in a common buffer which will be overwritten with subsequent executions.
Changed deviceMacAddress() to write its string into a user-supplied buffer, which is also returned.
Added .deviceIpByMacAddress(const uint8_t *mac), .deviceMacByIpAddress(IPAddress ip, char *sbuf) to all 3 archs
New MACAddress class, various device IP/MAC lookup functions rewritten to use it.
Fixed an instance of using IPAddress(0UL) instead of INADDR_NONE.
Merge pull request #480 from energia/issue_480

AP mode: Track client connects & disconnects - Had @robertinant review the code.