Not selecting buttons is not allowed when enableMultipleChoice is false

Hi @jlhonora.

I'm facing a situation where I need to be able to have no buttons selected when enableMultipleChoice(false).

I realized that situation is only possible when enableMultipleChoice(true).

So I've made some changes to reach this goal and if you think it is an acceptable change to the project fell free to accept this pull request (#47) and release a new version of the library.