Fixed bug in short-circuit detection for MEGA

The short-circuit code polled the track current every 10
“milliseconds.”  But Arduino millis() is based on TIMER-0 settings
which are changed in DCC++ Base Station.  The actual polling was closer
to every 1 millisecond.  When Base Station for the Mega for updated to
utilize TIMER-3 instead of TIMER-0, the millis() function now really
did count milliseconds, and 10 milliseconds between each track-current
sampling was too long.  Changed the code so that for the Mega, the
current sampling time is defined as 1 (instead of 10), representing an
actual 1 millisecond as desired.

Clean-up #include for Ethernet Library

This update cleans up the way Ethernet Libraries are included.  It is
designed to be compiled under the latest Arduino IDE - version 1.6.6.
This latest version of the IDE correct a long-standing “bug” in which
#includes for libraries must be present in the main sketch file, even
if not used in that file.  With the new version, the Ethernet Library
is named in only one place: Config.h.  Also, the library is not even
included if you compile for the Serial Port only (and therefore the
name does not need to point to any real library).

NOTE:  This version will ONLY compile with the latest Arduino IDE 1.6.6
that contain the needed “fix.”

Added auto-configure for Uno/Mega as well as Motor Shield Type

* board type detected at compile-time
* macro to select type of motor shield (Arduino or Pololu MC32926
* new <e> command that clears EEPROM
* safety check added to reject throttle command with nReg out-of-bounds
(<1 or >MaxNumRegs)

Updated