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