Adding a WaitForCondition command

We need to wait for things to happen in the client browser, so we have created a WaitForCondition, which just uses `browser.execute` to run the code in the client browser and wait until it is completed. 

There are options for a timeout and a waiting period, for we have found that sometime we need to wait a bit for the DOM to settle down. The waiting period is optional and set to 0 by default.