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)
• [x] 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
• [x] issue #29
• [x] issue #42
• [ ] issue #52
• [ ] https://github.com/gggeek/phpxmlrpc/issues/6#issuecomment-174355366

[gggeek]

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