Added some fixes and improvments

[ahorn42]

Hi Tobias,

I added some fixes to your mh-z co2 sensor library.

As an overview:

• quite irritating typo in the example (you mixed up pwm with serial)
• the response times where defined in milliseconds, but according to the datasheet it should be seconds
• I defined the preheat time as a constant as well (similar to the response time)
• I made it possible to initialize the sensors also with only pwm or only serial

If you want, you can add my github username to the authors list, but that isn't important for me ;)

I am looking forward to see at least some of this changes in the arduino library manager :)

best regards ahorn42