Added initial composer.json. WIP

It is detached PR #535 from my local master for the convenience of using branches
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@sandeepone Please add `cms` package  to `gleez` account: https://packagist.org/ and at the same time, check fields please. For fields available in composer.json see http://getcomposer.org/doc/04-schema.md

`@todo`:
+ Add Mango Reader
+ Add Cerber Theme
+ Add `suggest` field
+ Add `version` field