Shared runtimeenvironment

Added PHP execution logic to Verticle so its possible to keep the out stream open and the PHP Runtimeenvironment alive as long as the Verticle is alive, which should fix the echo bug https://github.com/vert-x/mod-lang-php/issues/1.

Also added QuercusContext creation logic to PHPVerticleFactory, to share the global context with all verticles created by this Factory. I did not test yet if this already works, what would hopefully fix https://github.com/vert-x/mod-lang-php/issues/2.

### ToDo:

- [x] think about a way to flush the WriteStream for each print/echo or at least for every handled message call.
- [x] test if "include modules" is working
- [ ] (maybe) create a WriteStream which writes everything into the logger