fixed bp_groupblog_get_user_role so that it gets roles for the target blog correctly

Coincidentally just saw your reminder ticket:

https://github.com/boonebgorges/bp-groupblog/issues/13

Not only does the function overwrite the `$blog_id` global, it assumes 'wp_' as the prefix, when this is in fact `$wpdb->base_prefix`.

BTW, sorry about including a bunch of other commits in the pull request - have yet to figure out how to exclude them.

Cheers,

Christian