Move time decoding into clock_generic.ino

The NTP library is finished when it has decoded the received
packet.  It does not need to go past epoch time for us.  Move
the epoch decoing into our generic clock handler so we have
better information about the whole clock time (hours, days, etc)
and so we can track clock skew over time.
Simplify time ticker console output

Instead of a clock-time every second, output a single
character each second with occasional markers of extra
activity.
Clean up some compiler warnings
Rename .ino -> .cpp except the one main file

The Arduino IDE seems to accept the .cpp files as easily
as the .ino files.  This change makes the main .ino file
easier for users to identify.  Maybe it also prevents the
IDE from concatenating files together into one, but
I didn't check.  Anyway, we do not need that anymore.
Add avr makefile using Arduino-mk

Add a makefile to perform similar builds as the Arduino
IDE from the command-line.  This uses the Arduino-mk
toolkit from https://github.com/sudar/Arduino-Makefile

There is a build error still unless the Arduino-Makefile
is updated so it does not include -I. in the include path
switches.  See
https://github.com/phord/Arduino-Makefile/commit/d998de8d84a29f1c7d5e1d9c541051ab29950b47
Move files to appease arduino IDE

The Arduino IDE insists that a .ino file be stored in a folder
bearing the same name.  This project is named 'master_clock' and
the .ino file is named 'master_clock.ino', so this has worked for
me so far.  But when the .zip file is downloaded from github the
top-level folder is renamed slightly.  Also a user may rename the
project folder when the project is cloned thus making the project
unbuildable via the IDE.

Move the arduino files into a subfolder named 'master_clock' to
match the master_clock.ino filename.  Adjust the makefiles and
.gitignore files to match the new layout.

Also fix the .ino file to include the library headers so the IDE
will notice the paths are needed when compiling the .cpp files.