Add CO2, eCO2 sensor types

[brentru]

Resolves #44

[caternuson]

Can leave library.properties unchanged. Whoever comes along to make the next release can deal with that.

Totally cosmetic - should it be all caps?

SENSOR_TYPE_ECO2

[caternuson]

oops. nvm. been merged.

[brentru]

@caternuson Will address in branch i'm checking out,

Totally cosmetic - should it be all caps?

Yeah I believe so, will update this here: #46

I'm going to cut a release shortly so I'd like .properties updated. For next time - are we leaving .properties alone for non-urgent releases?

[caternuson]

In general, yah, I think leave library.properties alone in the actual PR. Sometimes a release may be multiple PRs merged in no particular order. Or other reasons that may warrant a different sem ver number. And it's trivial to edit library.properties right before an actual release (i just use github's web edit).