Change bulkUpdate to return the affected rows

In order to avoid selecting the same data after performing a bulk update, we noticed that the update query [already returns](https://github.com/sequelize/sequelize/blob/f96f2f6b252cb052c2fb8da8f7de6ea11cd10e5f/lib/dialects/abstract/query-generator.js#L206-L209) the affected rows.

With this PR, executing an update will now receive the actual affected rows instead of just the row count.

This is related to issue #921.
This should have been a separate commit but was required to run the `update` tests (the ones added by this PR), since the returned object was not correctly parsing all *HSTORE* fields. This was fixed by [this line](https://github.com/sequelize/sequelize/pull/1624/files#diff-7e347c4738c97261969fbeb802a3c363L815).

I took the opportunity to simplify the HSTORE datatype definition into a string, since we didn't really do anything in the [previous function](https://github.com/sequelize/sequelize/pull/1624/files#diff-696825d45557dfe5b68972fd6f69587bL462).