canvas: add support for passing value to cgi for collection

engine.js: add support for passing value unit to cgi for canvas test
save_methanol_data.py: add support for parsing value and unit passed
from url for canvas test

Signed-off-by: Yongqin Liu<yongqin.liu@linaro.org>
Signed-off-by: Gabor Loki <loki@inf.u-szeged.hu>