fix PGCMD_NOANTENNA checksum

PGCMD_NOANTENNA checksum was wrong.
Add #include <SPI.h>

Necessary to explicitly include SPI.h since updates to the SD library for Arduino Due support
Add support for Arduino Due

Conditional compilation for Arduino Due.
Due support is hardcoded for Serial1
Added due_parsing and due_shield_sdlog examples
Merge commit '2a52767b76a5a9df15e26b83ea316b5116cc6fa6'
Due support
Merge branch 'master' of https://github.com/adafruit/Adafruit-GPS-Library

Conflicts:
	Adafruit_GPS.cpp
fix bad commit
Fixed Leo Locus examples

Restored interrupt to leo_locus examples, since this is required for
locus logging to work.
Correcting name of download button

The name of download button has changed - we correct this in the README.txt file.
Merge pull request #27 from nubok/patch-1

Correcting name of download button
Merge commit '5f0415ee68876e1ef1a7b13f9d835a496475a39e'

* commit '5f0415ee68876e1ef1a7b13f9d835a496475a39e':
  Correcting name of download button
  Fixed Leo Locus examples
  fix bad commit
  Due support
  Add support for Arduino Due
  Add #include <SPI.h>