[PowerShell] Enhancements to PowerShell generator

Description

Here are some enhancements for the PowerShell generator:

• Option to specify ClientPath (which is hard coded to "$ScriptDir....\csharp\SwaggerClient" at the moment"
• Integration tests using AppVeyor (similar to other API clients, we want to perform testings with the Petstore server (local) to ensure nothing breaks after changes)
• Review authentication support: HTTP basic, API key, OAuth2
• Add auto-generated documentation for API endpoints and models
• Update auto-genreated test files (APIs, models)
• Review README.md to see if the installation instruction, etc are clear

Swagger-codegen version

Latest master

Suggest a Fix

If anyone wants to work on these enhancements or have other suggestions on making the PowerShell generator better, please reply to let us know.