Ditch PHP 5.2 support, fix composer support, add syntax highlighting to readme, (almost) fix sample-project

Hey there,

In issue #22, you told me

```
We'd like to get rid of this stupid two uploadcares in a repo thing and ditch 5.2 altogether.
Not sure how it will affect composer.
```

So here is what I think it should look like.
Unfortunately, I havn't been able to make all tests pass since the credentials of the demo account are not working (anymore?)

https://travis-ci.org/vespakoen/uploadcare-php/jobs/19226194

The sample project runs fine over here, except for some stylesheets that are missing (you guys should probably fix this too, I get 403 forbidden responses

![dead-links](https://f.cloud.github.com/assets/876117/2213915/19ad136c-99c7-11e3-95ef-bc9323c45a83.png)
)

Please review and let me know what you think.