Display asterisk on checkbox widget

If the option `mopa_bootstrap.form.checkbox_label` is set at `widget`
when `mopa_bootstrap.form.render_required_asterisk` is `true`, asterisk
is not display, so I add it.
Merged conflict using new version
Merge branch 'wip-bootstrap-3-integration' of git://github.com/phiamo/MopaBootstrapBundle into wip-bootstrap-3-integration