Check for multiple extension when looking for driver service executable

Description
===========
Just like the Java client, the .Net client should not be strict about
the extension of driver service executable.
This push make it possible to execute chromedriver.{exe|cmd|bat|com}.

Reason
======
Because when using chocolatey (apt-get for windows) and the ChromeDriver2 package, the executable available in path end with ".cmd". This is due to chocolatey design.
And as the Java client look for multiple extension I thought it would make the .Net client consistent with the Java client.

Benefits
========
With this patch you can use chocolatey package and Selenium client without changing any configuration. This is very usefull to deploy selenium on server.

Drawbacks
=========
None?

I've signed the CLA.