API Cleanup

[gggeek]

At some point going forward, it might be worth doing a refactoring pass to modernize the existing API according to the usual oop best practices.

In no particular order:

• use exceptions for invalid requests/responses
• drop singleton-ness of helpers in favour of "proper" DIC; add interfaces for all helpers
• introduce class constants for all existing 'magic' strings
• add a Fault class ?
• drop long-deprecated methods and functions
• rename and simplify class internal members (eg. for Value and XMLParser)
• move most public class members to protected access
• move parseResponse method from the Request to the Client ?
• move Encoder::decodeXml method to the XMLParser ?
• allow for better parsing of big messages in memory-constrained cases by using streams by default instead of strings
• issue Accessing generated XML #29
• issue Please support more internal encodings than those of the xml parser. #42
• issue Bump php-version to 5.6 / 7 #52
• Roadmap? #6 (comment)

[gggeek]

NB: there is also the rewrite going on in the "future-ng" branch...