Drop support for PHP 5.2...

...and switch to namespaces (PHP 5.3+).
This also changes some of Plop's API and adds a new class & interface
for string interpolation (\Plop\InterpolatorInterface and
\Plop\Interpolator\Percent).
Update the documentation.
Merge branch 'php53'

Switch to PHP 5.3+ namespaces.