Use bp_locate_template to find best template

bp_locate_template() searches old template structure (/groups/, etc at
root of theme) and new style (/groups/, etc in /buddypress/ or
/community/ folder at root of theme). Does require BP 1.7+, though.
Provide pre-1.7 BP compatibility
Merge pull request #100 from dcavins/template-locate

Use bp_locate_template to find best template to use