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()
Update strings for 2.0.0-dev.2
Fix missing quote
Merge latest strings (2.0.0-dev.2) to existing Locale files
Update strings for 1.5.8
Merge latest strings (1.5.8) to existing Locale files
Merge branch 'master' into 2.0
Update strings for 2.0.0
Merge latest strings (2.0.0) to existing Locale files

This was merged with -N (fuzzy) enabled
Translated using Weblate (Dutch (ISO 639-2))

Currently translated at 44.5% (334 of 750 strings)
Translated using Weblate (Dutch (ISO 639-2))

Currently translated at 46.5% (349 of 750 strings)
Translated using Weblate (Dutch (ISO 639-2))

Currently translated at 47.6% (357 of 750 strings)
Merge branch 'pr/32'