Added "printer discovery"

Added some threads to listen to the broadcast UDP by the printer to pass
the IP of the printer, so the to-be-printed document can be sent to the
correct printer
Added a thread to check whether the IP disappears

A thread is added to check whether the printer or connected AP is gone (IP
has disappeared)
Added a thread to check whether the IP disappears

A thread is added to check whether the printer or connected AP is gone (IP
has disappeared)
Merge commit 'd7dc80' into HEAD

Conflicts:
	src/com/example/printer/DisplayPDF.java
	src/com/example/printer/DisplayPic.java
	src/com/example/printer/ListenUDPBroadcast.java
	src/com/example/printer/ResponseCountdown.java
Fixed bug for printing PDF documents

Fixed a bug when displaying PDF so it can provide correct IP of the
connected printer when user chooses to print the document
Make it only display pictures with type jpeg

Current printer only can print out jpeg and PDF, so filtering is needed to
only display jpeg files
Replaced sending messages to broadcasting signals

Using broadcast-receiver to broadcast the IP status change instead of
transferring messages via handlers to reduce the amount of thread created