This patch improves phantom_shim:
- introduce WebPage.loading/WebPage.loadingProgress
- improve compatibility of WebPage.onInitialized. This allows to
pass phantomjs tests, even though the implementation is hacky.
- teach PhantomResponse about "stage" state which could be either
"start" or "end"
- unskip a bunch of phantom webpage tests:
- webpage/change-request-encoded-url
- webpage/loading.js
- webpage/long-running-javascript.js
- webpage/modify-header.js
- webpage/on-initialized.js
- webpage/remove-header.js
