dropdown block description
button: fixed extra padding in FF
button_disabled: icons must be as gray as text
Changes to documentation
buttom.ru.md
Merge pull request #513 from bem/gela-d.changes-to-documentation

Changes to documentation
button_type_link: added `_disabled` example
button: checked+disabled state implemented (close #380)
dropdown.ru.md
Merge pull request #512 from bem/issues/#498@v2

dropdown block description
button_type_link: don't add `href` attribute if disabled (close #520)
Merge pull request #521 from bem/issues/520@v2

button_type_link: don't add `href` attribute if disabled (close #520)
button.en.md added
Merge pull request #515 from bem/button-doc-en

button.en.md added
base-control english version + Coltspb review