Codeception issue #1510 - site-root relative URL handling

Changing Guzzle::getAbsoluteUri to handle site-root relative URLs.  This
changes how PhpBrowser handles relative URLs everywhere, including
amOnPage, click(), submitForm, etc...
Updating WebDriver amOnPage to handle relative URLs

WebDriver was handling site-root relative URLs in the same way as
doc-relative.  This has been updated so a site-relative URL is relative
to the domain specified in the 'url' configuration setting.
Cleaner code to match relative URLs

Building the URL with PHP's parse_url then passing it to
\GuzzleHttp\Url::buildUrl
Secondary code cleanup

Sorry - silly first commit had lines that could be further cleaned on
second thought.
Updated TestsForWeb test to use existing page

FrameworksTest fails with a non-existent URL - updated the last amOnPage
in testSiteRootRelativePathsForBasePathWithSubdir to use a real page.
Form doc-relative action fix, app-relative URLs

Document-relative URLs for submitForm now fixed, and added test code.

Moved 'basePathWithSubdir' test to TestsForBrowsers since it doesn't
make sense for Frameworks and was causing a 'risky test' message.

Updated PhpBrowser added amOnPage so it strips out leading
forward-slashes, making URLs app-relative for amOnPage.  Fixed test
code so it matched.
Merge pull request #1514 from zbateson/relative-site-root-paths-urls

Codeception issue #1510 - site-root relative URL handling