Update readme.md

Allow 5.2

update readme for 0.6.x

Add L5.1 for 0.5.x

Allow 5.1

Fixes barryvdh#124

Tweak config

Add phpdoc

Use PSR-4

Resolve new instance in Facade

Instead of in the wrapper. Should create new instances for
App::make('dompdf') and PDF::loadHTML() calls, and be chainable again.

Tweak ServiceProvider

Pull the defines etc outside the dompdf binding. Will only be run when
dompdf is requested, because of deferred.