Add autoWebview capability

Milestone 1.2 issue #2715

Allow user to enter directly into webview context. Adds universal capability `autoWebview` (`true`/`false`) and Android capability `autoWebviewTimeout` (in seconds, defaults to `2`), to set hot long to wait before checking for the webview. 

Also fixes Android/Selendroid webview tests.
I actually think this timeout should be in ms. Most of our timeouts are, except for a couple that are in seconds. I'd like to eventually standardize on ms. What do you think @bootstraponline?