Revert "Support BP_ROOT_BLOG for BuddyPress sites. Fixes #88"

This reverts commit 824af4f22bbe4cf1799f6efcb1ece3a77dea47f4.
Revert "Revert "Support BP_ROOT_BLOG for BuddyPress sites. Fixes #88""

This reverts commit 3d96c4d24181b11bae08c5818f736970c89e1512.