Use composer compoenents

## What changed

* Old `Package`, `Converter` and `Parser` classes have been replaced with their composer counterpart (`CompletePackage`, `Loader`, `Dumper`)
* `pickle.json` format has been changed to look more like `composer.json` and is validated using a [JSON schema](https://github.com/jubianchi/pickle/compare/composer?expand=1#diff-10)
* During package conversion, we do not generated `RELEASE-*` files anymore : only the last release is kept in the `pickle.json`
* Other files are not anymore generated:
    * `LICENSE`: the license is kept inside the `pickle.json`
    * `README`: the extension description is taken from `/package/summary` in the XML and kept in the `description` field of the `pickle.json` file
* `install` and `info` commands now take an argument which can be:
   * a path to a local folder holding the extension
   * a PECL extension name: `pickle install apc`
   * a PECL extension with stability constraint: `pickle install apc-beta` (default stability is `stable`)
   * a PECL extension with a version constraint: `pickle install xdebug@2.2.5`