3.0.0: Simplification

- Bumped PHP version requirement to 5.4+
- Removed old Response namespace and associated classes
- Added new "CurlPlusResponse" class for all responses
- Removed "CurlPlusClientState" class.  The states are now constants on the "CurlPlusClient" class itself
- Added dependency "dcarbone/curl-header-extractor" to facilitate header/body separation from responses
- Minor documentation and test updates to reflect changes
- Removed Autoloader
- Removed all Interfaces.  This lib is simple enough.