Fall back to use http stream when Curl module is not enabled

Many hosted environments and custom platforms do not have the PHP Curl module enabled. One very popular example is the Google App Engine. In these environments, the official SDK becomes unusable and developers have to rely on unmaintained forks or write their own custom code. This pull request tries to address this issue by falling back to use http streams if curl is not present. Having support for http streams (as a fallback) can make the official php sdk more widely used. 

I've tested the commit on environment without curl, and all the unit tests pass. 

If we agree on adding this to the official sdk, I will be more than happy to work on any feedback or suggestions that you may have to make this better.

Thanks!