Bump version numbers in trunk to 2.1-alpha

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8283 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Add imath to contributor list in readme

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8284 cdf35c40-ae34-48e0-9cc9-0c9da1808c22