Fixing Window sizing tests for Firefox 26 on Windows.

Starting with Firefox 26, the minimum width of the window is ~405 pixels.
The tests attempted to set the window width to 275 pixels, which would not
succeed. We now use 450 pixels as the width to set to.
Adding experimental IWebDriver extension methods to .NET support classes

This commit adds extension methods to IWebDriver, which can be used to
reduce the amount of type casting that must be done to access
functionality not provided by the base IWebDriver interface. The following
methods were added:

 * TakeScreenshot() - does not require a cast to ITakesScreenshot, and has
   support for working directly with instances of RemoteWebDriver.
 * ExecuteJavaScript<T>() - does not require a cast to
   IJavaScriptExecutor, and will automatically cast the result to the type
   specified by the generic type parameter (T).

These extension methods should be considered experimental. To use them
requires a reference to WebDriver.Support.dll, and adding a using clause
for OpenQA.Selenium.Support.Extensions.
FxCop and StyleCop fixes. No functional changes.