[PHP] Symfony generator enhancements

[ksm2]

PR checklist

• Read the contribution guidelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

Hi, after an exciting G20 weekend here in Hamburg, Germany, I finally implemented the following improvements to the Symfony generator for #5985:

• Use camelize instead of initialCaps
• Use File.separator instead of "/" to make it work in the Windows platform
• Better README with installation instructions, how to start, etc
• Write an options test
• Fixing codegen errors in the models
• Parsing header, path, query, and form parameters
• Generate setters for auth methods to handle them accordingly
• Change "getters" to "issers" for boolean properties (Symfony standard)