Add composer.json

Squashed commit of the following:

commit 9880b1a9d9db7a980ca6ec49ab93bb003bc4338f
Author: Hadrien Collongues <chadrien@chadrien.fr>
Date:   Thu Feb 13 16:26:31 2014 +0100

    Add composer installation doc in the README

commit 11b3cee4113f5a395613cef606050f26ef6c1427
Author: Hadrien Collongues <chadrien@chadrien.fr>
Date:   Thu Feb 13 16:16:53 2014 +0100

    Use 2.0 branch, not 2.x tag

commit 745c3c1684bc5e085badfd2b793e64336be48b66
Author: Hadrien Collongues <chadrien@chadrien.fr>
Date:   Thu Feb 13 16:14:24 2014 +0100

    Added composer.json for composer installation

    Combined with an App::build()