Added support for limit clause on update

This PR adds <code>limit</code> option to Model.update (related to https://github.com/sequelize/sequelize/issues/1666)

It allows to make queries like this :
<code>UPDATE Resources SET userId = 1 WHERE userId IS NULL LIMIT 1</code>
via Model.update


Only mysql and mariadb support limit on update.
Postgres doesn't support it.
SQlite supports limit on update only if it is built with the SQLITE_ENABLE_UPDATE_DELETE_LIMIT compile-time option.