Method for loading fixtures from file added.

This PR allows to load fixtures from a single file by adding `loadFromFile` method to `Loader` class.

In some projects which require various fixture sets (e.g. for different setups for various environments) being able to point fixtures only by directory is quite a pain as some fixtures may be reused in few sets. Moving them to subdirectories doesn't solve the problem in some cases as namespaces would have to be adjusted accordingly. There were some issues reported concerning the lack of this feature both in this repo and in DoctrineFixturesBundle. They didn't however contain any code needed to accomplish the goal.

I am planning also to provide a related PR to DoctrineFixturesBundle when this one meets your acceptance.

PHPUnit tests fail in one case, but it does not seem related and it fails also on `master`. Namely:
```
There was 1 failure:

1) Doctrine\Tests\Common\DataFixtures\ProxyReferenceRepositoryTest::testReferenceReconstruction
Failed asserting that Doctrine\Tests\Common\DataFixtures\TestEntity\Role Object (...) is an instance of class "Doctrine\ORM\Proxy\Proxy".

/home/mariusz/Development/data-fixtures/tests/Doctrine/Tests/Common/DataFixtures/ProxyReferenceRepositoryTest.php:116
```
Well, if you look at the original code, you will see that this is the same way it was done for directories. And loading fixtures is not something you do every 5 minutes, so even if it takes few seconds I don't consider this to be any problem.