Add support for generating measurement data.

This patch adds a new test case `measure.exp` for generating energy consumption
data in csv form.

It relies on environment variables as defined in the `run-avr-measurement.sh`
script.

~~~
Usage: ./run-avr-measurement.sh [--target-board <board>]
                                [--avrdude <program>]
                                [--avrdude-flags <val>]
                                [--energytool-serial <val>]
                                [--energytool-pin <val>]
                                [--energytool-point <val>]
                                [--energy-data <file>]
                                [--timeout <val>]
~~~

	* beebs.test/measure.exp: New test case for measuring.
	* run-avr-measurement.sh: New script for easily running measurements of
	all the benchmarks.
	* tools/energytool.py: New python script for taking one measurement. It
	relies on pyenergy.