Major performance improvements in CURL.
Major performance improvements in CURL.

The CURL transport underwent minor surgery to do two things:
- Reduce calls to curl_setopt()
- Use curl_multi_* for connection pooling.

The later made a huge difference, cutting the amount of time
required for running unit tests by 70%.

hpcloud/HPCloud-PHP,hurongsheng/gokuai-swift,programster/openstack-sdk-php,ktroach/openstack-sdk-php,gokuai/gokuai-swift