Add an "envinfo.php" script to help diagnose environment problems
Summary: Wrote this for https://github.com/facebook/phabricator/issues/98, seems worth keeping around.
Test Plan: Ran script, got useful output.
Reviewers: nh, btrahan
Reviewed By: btrahan
CC: aran, epriestley
Differential Revision: https://secure.phabricator.com/D1931