PHP5.3+, PSR-4, Namespaces

This PR converts EasyRDF into namespaced library. 

## Changes

* PHP 5.2 is not supported anymore. New `composer.json` mentions `5.3.0` as minimal compatible version. In practice, number is a bit higher because of dependencies.
* Usage without composer is not supported anymore 
* Library is loaded via PSR-4 autoloader now
* Classes are renamed like this: `EasyRdf_Parser_Turtle` → `EasyRdf\Parser\Turtle`
* With a single exception: `EasyRdf_Namespace` → `EasyRdf\RdfNamespace` (because `namespace` is a keyword in PHP)
* PHPUnit is updated to recent 4.1 series
* ARC2 2.2 is added to `require-dev` and `suggest` sections of `composer.json` (so tests for it are always run).
* EasyRdf expects HTTP-client objects compatible with ZendFramework 2.x instead of 1.x now. (`zend-http` is added to `require-dev` so tests for it are always run)

## Check list:

- [x] Convert library-classes to use namespaces
- [x] Use PSR-4 autoloader
- [x] Remove php52 fence-files around JsonLD Parser/Serialiser
- [x] Convert unit-tests to use new namespace-based names of library classes
- [x] Convert "acceptance tests"
- [x] Make sure that tests do not fail
- [x] Convert examples
- [x] Update docblocks