Version 0.6

Rundown of changes:

* Performance enhancement - Locally-cache lookups and reference cache before doing another lookup
* Performance enhancement - Switch from preg_replace() to substr_replace()
* Support BP username compatibility
* Support WP Toolbar
* Support bbPress
* Better support for 'BP_SHOW_DISPLAYNAME_ON_PROFILE
 constant
* Fix 'What's New' string in activity post form
* Requires at least BP 1.5
Bump stable tag to 0.6.
During a search in the members directory, search against the username
column as well.

Only works in BuddyPress 1.7 and up.

Fixes #2.
Update readme.txt.