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