Allow the user to set a custom time zone

As mentioned in #1690.

This allows the user to provide a parameter to the sequelize constructor with a timezone offset. This offset will be used for two things:

* When connecting, we do SET TIME ZONE, which affects the output of functions such as NOW
* When converting a DB date to a javascript date we use the time zone offset. This only applies to mysql, since postgres and sqlite (> dev12), stores the timezone in the date string.