MakefileExample: Fix AVR_TOOLS_DIR default

Judging by the default for OS X directly above it and the fact that the current default doesn't work, it seems this should not include the `/bin`.
Merge pull request #237 from bgamari/patch-1

MakefileExample: Fix AVR_TOOLS_DIR default
allows "make clean" to be extended in the user's local makefile to clean
other files out, e.g. backup/git files
Merge pull request #240 from sej7278/master

allows "make clean" to be extended

Fix #239
Fix #240
Fix regex for "make show_boards" on 1.5

In 1.5.6 the Due board has an underscore, this regex tweak allows for:

arduino_due_x_dbg  Arduino Due (Programming Port)
arduino_due_x      Arduino Due (Native USB Port)
Merge pull request #241 from sej7278/master

Fix regex for "make show_boards" on 1.5
Small typo, can lead to copy and paste fails

Signed-off-by: Joseivaldo Benito Junior <jrbenito@benito.qsl.br>
Merge pull request #247 from jrbenito/master

Fix a typo in README