Override complete compiler tool paths for chipKIT.

Previously we were setting the AVR tools path to the PIC32 tools path because it
made grabbing the compiling tools easier. Consequently, it made finding the
avrdude path much harder, especially since the avrdude files are in different
locations in the Linux distributions of MPIDE and Arduino.

Instead, we set the AVR tools path to the *correct* path (where *AVR* dude
lives), and totally override the CC, CXX, etc. paths to point to their PIC32
equivalents.
Silence the stderr output from call to `which`.
Don't hard code MONITOR_PORT in examples, for more flexible testing.
Use more reliable serial device naming in Windows.

* Strip leading "/dev/" from MONITOR_PORT before handing to avrdude in Windows.
* Use the more widely available awk tool instead of bc to subtract 1
    from COM ID (as opposed to `bc`).
* Allow Windows user to specify "com1" or just "1".
* Document MONITOR_PORT format for Windows users.
Document that ARDUINO_DIR must be a relative path in Windows.
Merge pull request #155 from peplin/monitor-port-check

Use more reliable serial device naming in Windows.
Document that ARDUINO_DIR must be a relative path in Windows

Fix #156
Merge pull request #157 from peplin/no-hardcoded-port-in-examples

Don't hard code MONITOR_PORT in examples, for more flexible testing.
Merge pull request #158 from peplin/silent-which

Silence the stderr output from call to `which`.
Merge pull request #159 from peplin/simpler-crossplatform-paths

Override complete compiler tool paths for chipKIT.
Update changelog about the recent changes
Update `README.md` about compatibility with Windows

Fix #94
Fix some typos and errors in changelog
Bump up version to v1.3.0 for release
BUNDLED_AVR_TOOLS_DIR is now set correctly using := instead of ?=,
so that installations using only the arduino-core packages that
don't have the $(ARDUINO_DIR)/hardware/tools/avr directory can
still use the avr-g++ tools found in the $PATH (/usr/bin)

Previously BUNDLED_AVR_TOOLS_DIR was set to an empty string as
the directory doesn't exist, which meant that "ifdef BUNDLED_AVR_TOOLS_DIR..."
was set rather than skipping to "else SYSTEMPATH_AVR_TOOLS_DIR...."

No user would set BUNDLED_AVR_TOOLS_DIR so the assignment operator
:= should be used not ?=

Updated the version info/changes in various locations.
Merge pull request #161 from sej7278/master

BUNDLED_AVR_TOOLS_DIR is now set correctly
Document all the variables that can be overridden in child makefile

Huge thanks to @sej7278 for doing this

Fix #75
Add a new target `help_vars`

This target can display all variables that can be overridden
Bump up version to 1.3.1 for release
Added the arduino-mk-vars.md file to the Fedora SPECfile.

Added some notes to the SPECfile regarding overriding the paths
to the avr tools - most are in /usr/bin, but gcc/g++ are in an
architecture-dependant ccache directory

Upped version to 1.3.2 devel