Improved extends and $ref support, added json-schema-test-suite test-cases

I integrated the [json-schema-test-suite](https://github.com/Julian/JSON-Schema-Test-Suite) test cases.
Fixed about 20 tests but still some fail.

```
$ ./vendor/bin/phpunit 
PHPUnit 3.7.19 by Sebastian Bergmann.

Configuration read from /Users/janmentzel/work/json-schema-php/phpunit.xml.dist

...............................................................  63 / 452 ( 13%)
............................................................... 126 / 452 ( 27%)
...........................................................F... 189 / 452 ( 41%)
.......F....FFF.FFFF.F..FF........F...........FFF.............. 252 / 452 ( 55%)
.....F............F..........................F....EEEEEEEEEEEEE 315 / 452 ( 69%)
.....FEE....................................................... 378 / 452 ( 83%)
..................................F.F.F..FEE................... 441 / 452 ( 97%)
........F..

Time: 1 second, Memory: 17.25Mb

[...]

FAILURES!
Tests: 452, Assertions: 464, Failures: 25, Errors: 17.
```
For its remote schema tests the testsuite needs a small http server running on http://localhost:1234
Please see README.md for how to install and run.

If you don't want the json-schema-test-suite tests to run, simply comment out the code in `test/SuiteTest.php`.