Make $select.multiple definition more readable.
Support `multiple="multiple"` attribute.

Some html preprocessors will transform

<ui-select multiple></ui-select>

into

<ui-select multiple="multiple"></ui-select>

which is currently not supported by angular-ui-select. This patch fixes the
issue.
Merge pull request #407 from ctavan/allow-multiple-multiple-attribute

Support `multiple="multiple"` attribute.