Window position and sizing

This pull request adds the methods for positioning and sizing a Window as spec'd by the [window_switching_spec](https://github.com/watir/watirspec/blob/aab747291a46c85a92f71601073daab1381c6d91/window_switching_spec.rb#L265-L323).

Note that the window_switching_spec required modifications to verify that these tests pass. The spec has windows being located using two parameters (how, what) as opposed to a single hash. The window method currently only supports a single parameter, which causes the test failures. An additional pull request will be created to address this other issue.