Default to openssl_random_pseudo_bytes instead of /dev/urandom.

Summary: When available, prefer php's built in openssl_random_pseudo_bytes() to /dev/urandom. Test for functionality and fallback to /dev/urandom if necessary to get data. Simplifies the code a bit, and is somewhat more functional as running inside a chroot is now possible.

Test Plan: Add unit test for readRandomBytes() Would be nice to have someone test this on windows. Working on ubuntu at the moment.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7528