Bring `bp_get_blog_avatar()` up to speed with `bp_get_group_avatar()`:

* Add inline doc
* Use `bp_parse_args()`
* Add brackets and bring code formatting up to date

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8759 cdf35c40-ae34-48e0-9cc9-0c9da1808c22