Working towards next release

- Adding new option property "wsdl_cache_path"
- "wsdl_cache_path" is now a static public class property on SoapClientPlus
- No longer differentiating between "temp" and "cache" files, setting flag for removal on destruction if cache is not set to 'true' in some fashion
- ICurlPlusContainer interface methods now properly return "$this" in keeping with interface design
- Exception messages have been modified to (hopefully) be more useful
- "parseXML" has been moved to be a class-level protected method
- Some minor improvements with the goal of improving performance slightly
- Upgrading to dcarbone/curl-plus 0.6.*