Handling of protocol relative urls

Problem/Motivation

advagg_install_url_mod() and advagg_glue_url() seem to have trouble to handle protocol relative URLs.

Proposed resolution

Attached patch introduces a special handling for protocol relative URLs.

Since advagg_install_url_mod() has to return an url that can be handled by drupal_http_request() we need to specify a scheme and can't use the protocol relative path.
To do so the adjusted code uses $base_url.

advagg_glue_url() should now understand protocol relative URLs and not end up creating //:// for them.

Remaining tasks

Reviews needed.

User interface changes

None.

API changes

advagg_glue_url() now maybe returns a protocol relative instead a relative path.

Data model changes

None.