Issue #1512818 by infojunkie: Fix AJAX language URL issues.
Issue #1491782 by neilnz: ESI roles/user cookies do not use session.cookie_domain
Removing code from master branch; all work is in version-labelled dev branches.
Removed redundant debug code.
Added an admin-page to configure the ESI module; enabled ESI render-mode as an option; cleaned up variable_get defaults.
Switched from outputting a hard-coded ESI tag to using a render function, which allows the use of SSI tags and other render modes.
Added integration with the context module.
Added tests to check ESI handling in blocks - checks the URLs are created correctly, and the handler serves the correct block content.
The request_uri() function requires the page context in ['REQUEST_URI']. (TODO: add support for non-apache servers)
Corrected typo: should be 'public', not ''.
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