Add add_price() method and fix some syntax errors

Signed-off-by: Tomas Hozza <thozza@gmail.com>
Fix a syntax error (typo)

Signed-off-by: Tomas Hozza <thozza@gmail.com>
rename energy-track dir to energy_track

Signed-off-by: Tomas Hozza <thozza@gmail.com>
Swap timestamp and meter_value in Measurement constructor

Signed-off-by: Tomas Hozza <thozza@gmail.com>
Add __init__.py files in directories

Signed-off-by: Tomas Hozza <thozza@gmail.com>
Add unit tests for Measurement class

Signed-off-by: Tomas Hozza <thozza@gmail.com>