Change to HeatIndex function using built in DHT library plus set C or F

[cbrherms]

Now using the built in dht.computerHeatIndex function as found in the arduino DHT library.

This now allows celsius to work with HeatIndex (did not previously) so added definition to make setting fahrenheit or celsius simple, incorperating @rdbahm idea in to my own fix.
Fixes #28

Added note and fixed a few typos

[rdbahm]

I think this is a better way to implement the change than my way to do it - puts the logic in the upstream library and shrinks the size and complexity of the multisensor's code.

[theetron]

Thanks for this addition.

[bruhautomation]

Many apologies for the delay getting back to these repos! I've migrated away from this codebase after the esphomeyaml project was announced. I plan on marking these repo's deprecated soon because that project is vastly more capable.