Fixes programmatic handling of checkboxes

[noisecapella]

This is related to #58 and mostly adapted from this commit fixing the problem for Switch: 3078783

[tleunen]

Thanks @noisecapella. Could you please fix these eslint errors?

react-mdl/src/Checkbox.js
  27:9  error  Expected indentation of 6 space characters but found 8  indent
  29:9  error  Expected indentation of 6 space characters but found 8  indent
  33:9  error  Expected indentation of 6 space characters but found 8  indent
  43:5  error  Expected indentation of 2 space characters but found 4  indent

The others 2 errors are unrelated to this commit and I'll fix them later

[noisecapella]

Just pushed!

By the way it seems like similar changes would be needed for Radio and possibly other components

[tleunen]

Indeed, the other "input fields" could have the same issue.. I'll try to take a look in the next days