Merge proxy options into soapClientOptions configuration.

This ensures that the proxy configuration is used during both code generation and subsequent service calls.

Update phpunit requirement to ~4.4 as the updated test uses assertArraySubset introduced in 4.4.0.
Code cleanup

- Reformatting
- More error handling
Do not require the proxy_ prefix for proxy option keys.

proxy is already included in the option name. Instead we prepend it during normalization.
Add documentation of proxy option to README.