Check if native PHP function curl_init is available before use it

Hello Guys,

I was making some experiments with HipChat API through [this client](https://github.com/gorkalaucirica/HipchatAPIv2Client). This client has Buzz as dependency and it was not clear for me in a first moment that `curl_init` (native function) was missing (yes... just apt-get install php5-curl).

So this is a really small PR that might thrown a friedly exception. Considering that I've found `curl_init` in other parts of the code I've added `lib-curl` as required lib in `composer.json`.

Since it's an abstract class, static and protected method it becomes a bit hard for testing. However I've created a test class and put some small tests.

I don't know if these changes make sense just in some way, but here is my small purpose.

Cheers
using ``self::`` to access an instance method is a bad practice