Added downcaseAssociationsAccessors configuration option

Hello there, as i have already commented on this issue a few months ago ( http://github.com/sequelize/sequelize/issues/652 ), this is the second time where im facing this little issue regarding the default and not-configurable behavior of down casing the first letter on the associated models accessors when eager loading.
Example:
User.hasOne(Profile)

When including the Profile model in any query on the User model, all the profile data will be inside a 'profile' property, instead of the original 'Profile'.

I understand that this is a convention and/or default in SOME circumstances, however, when you have to deal with a currently working API, and the capitalization is important for the responses, having to create object reformatters is a little overkill when this behavior could just be disabled with a simple fix.

Ideally with this patch, you can have an result object that respects the capitalization of the associated models.

I would love this pull to be merged into master, please let me know if you think this is not possible as im currently using a custom patched version of sequelize in 2 projects because of this.

Thanks!