Fix: Switch to PSR-4 autoloading

[localheinz]

This PR

• [x] switches to PSR-4 autoloading
• [x] flattens the directory structure

Replaces #8, per https://github.com/In-Touch/newrelic/pull/14#issuecomment-145852075.

:information_desk_person: See http://www.php-fig.org/psr/psr-0/:

Deprecated - As of 2014-10-21 PSR-0 has been marked as deprecated. PSR-4 is now recommended as an alternative.

[lukewaite]

Rebased against master in #22 and merged. Thanks @localheinz!