Use toString function in README

This helps illustrate that the uuidX() functions return Uuid objects,
and not strings.
Set the default timezone to avoid errors in hhvm

Added a default timezone to the bootstrap.php to fix errors in HHVM
tests, which was introduced when Travis removed the default `/etc/hhvm/php.ini`
in https://github.com/travis-ci/travis-ci/issues/2523#issuecomment-59303763
Merge pull request #35 from jwpage/doc_toString

Use toString function in README