Added footer

Added footer with invalid link location
Pi

Replaced M- with Pi
The value for Pi is wrong and too short
Pi is appended to current display rather than clearing it
Demo limit functionality

Added demo limit for 50 button presses with an error message is which
has a number of issues:
Informal
Calculator is spelt wrong
Contains an !
Ends on a :
States the limit is based on operations not key presses
States the limit is 25 not 50
Once you dismiss the message you can continue
There is no where to entered prompted for serial number
Overflow error behaviour

Added overflow error behaviour
If the result is greater than 9999999999 then the message 99999999Err is
displayed and the display background turns red
The display background is only cleared on C or refresh
It is possible to start over new operations without clicking C
It is possible to enter more digits than a result can be
Added divide by zero error message functionality
Added readonly message

Added a readonly message to the display and history.  Memory doesn't
have a readonly message.  The fields aren't actually read only
Merge remote-tracking branch 'origin/Spa2014' into gh-pages