Fix information leakage by validating the URL scheme

Fix information leakage by validating the URL scheme

Validate the scheme to prevent leaking information by abusing the
file:// scheme.

Before this change it was possible to see what files are available on
the system running tweeper.

The script in tests/test_information_leakage.sh shows the problem on
earlier versions.

Here is an execution with tweeper-0.6:

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URL file://twitter.com//etc/passwd
--> /etc/passwd
    exists

URL file://twitter.com//etc/file_with_an_unlikely_name
... /etc/file_with_an_unlikely_name
    does not exist

Staring a test server

URL file://twitter.com//etc/passwd
--> /etc/passwd on http://localhost:8000
    exists

URL file://twitter.com//etc/file_with_an_unlikely_name
... /etc/file_with_an_unlikely_name on http://localhost:8000
    does not exist

Shutting down the test server
-----------------------------------------------------------------------

Here is an execution after this fix:

-----------------------------------------------------------------------
PHP Fatal error:  unsupported scheme: file in /home/ao2/Proj/Tweeper/tweeper/tweeper.php on line 323
URL file://twitter.com//etc/passwd
... /etc/passwd
    does not exist

PHP Fatal error:  unsupported scheme: file in /home/ao2/Proj/Tweeper/tweeper/tweeper.php on line 323
URL file://twitter.com//etc/file_with_an_unlikely_name
... /etc/file_with_an_unlikely_name
    does not exist

Staring a test server

URL file://twitter.com//etc/passwd
... /etc/passwd on http://localhost:8000
    does not exist

URL file://twitter.com//etc/file_with_an_unlikely_name
... /etc/file_with_an_unlikely_name on http://localhost:8000
    does not exist

Shutting down the test server
-----------------------------------------------------------------------