Overhaul: PHP namespaces and PSR-4 layout.
README updated for Pheanstalk v3.0.
build_phar.php updated for PSR-4 etc.
composer.json alias dev-master to 3.x-dev, was 2.x-dev.
composer.json more permissive for PHPUnit version.
PHP 5.6 added to Travis build.
Merge pull request #83 from pda/v3

V3: Pheanstalk namespaced, PSR-1, PSR-2, PSR-4.