adding command_executor to support multiple commands from a single shell

[mwrock]

Deprecates WinRM::WinRMWebService methods cmd, run_cmd, powershell, and run_powershell_script in favor of the run_cmd and run_powershell_script methods of the WinRM::CommandExecutor class. The CommandExecutor allows multiple commands to be run from the same WinRM shell providing a significant performance improvement whwn issuing multiple calls.

This PR is a prerequisite for WinRb/winrm-fs#28 which depend on CommandExecutor, which is being added here since it provides value beyond just transferring files.

[mwrock]

Note that I have removed the ShellCloser class used as a finalizer for CommandExecutor. While users of CommandExecutor are encouraged to call close explicitly, I think we can leverage the finalizer in a transparent manner upon GC.

I'd be curious to know if @fnichol has any words of caution regarding this strategy. I have tested GC's on the CommandExecutor as it is here and have not had issues with it losing sessions.

[mwrock]

I just made a few additional changes here:

• Moved the retryable method from the fs-transport winrm-fs branch into the CommandExecutor. Made more sense for that to live here and its a nice feature to have for openning shells.
• Expose the WinRMWebService logger via a attr_reader and set its level to :warn and add the stdout appender on initialization. WinRMWebService consumers can access its logger and change level, appenders, etc. This will be helpful when it is plugged into Test-Kitchen and make it more natural to integrte the winrm logging into the test-kitchen log streams.
• Added a convenience method to WinRMWebService: create_executor. This can be used 2 ways:

winrm.create_executor do |executor|
  executor.run_cmd('ipconfig')
end

This ensures the executor is closed when done.

executor = winrm.create_executor
executor.run_cmd('ipconfig')
executor.close

[mwrock]

I think this is ready to review. @sneal let me know if you think there is more to change or if I'm off anywhere.

[sneal]

@mwrock Awesome stuff here, I love seeing things getting pushed upstream for reusability. I'll take a good look later next week when I have my laptop.

[sneal]

whwn => when

[sneal]

[mwrock]

rebase/squashed and removing 1.5 versioning artifacts. Will merge that in a separate PR