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
this check should be performed in the constructor actually if you want to check it explicitly