Add initial implementation of scheme handling

This only supports WebKit1 and is based on keis' commit:

commit 7c7e4daa1d175a2a7ca0eaf26605cadca35640e8
Author: keis <keijser@gmail.com>
Date:   Sun Jun 19 15:35:04 2011 +0200

    add dummy request handler

which no longer worked due to changes in libsoup.
Implement a scheme command

This adds a custom scheme support to uzbl. It is based off of keis'
commit:

commit 8b81b5d86f9c98ddc6161bda6534d36ea05dba8b
Author: keis <keijser@gmail.com>
Date:   Sun Jun 19 19:03:18 2011 +0200

    add command to configure request handler

    * possible to associate any scheme with a command

and modified to support the new dev/cleaup patterns. One major change is
that "scheme foo cmd" instead of "scheme foo = cmd" is used.