Add support empty value of select's option
set nb.button disabled on init
Merge pull request #102 from yandex-ui/nb-button-disabled-oninit

set nb.button disabled on init
Merge pull request #99 from yandex-ui/nb-select-empty-option-value

Add support empty value of select's option
Select addToSource() removeFromSource()
Select addToSource() removeFromSource()