make eco2 + tvoc properties to match sensor standard

[ladyada]

ed1db09#commitcomment-25187627

[sommersoft]

To clarify, change all property instances of tvoc to TVOC, and eco2 to eCO2.

Outlined in the design guide here.

Like so:

# old
@property
    def tvoc(self):

# new
@property
    def TVOC(self):

[ladyada]

yep thats it - super easy :D but have to update examples, readme too!

[process1183]

I'd be happy to work on this one!

[ladyada]

@process1183 go for it!