[New Command] Licenses of dependencies

At [Alter Way](http://www.alterway.fr/), we need an easy way to check the correct licensing of a project in its entirety; this include checking that every dependency actually has at least one license, that none of these licenses are incompatible with the policy of the final product (e.g. no Affero GPL), etc.

We created a command that generates the list of the licences of the whole tree of depencies used in the project. It currently provides two formats: an easily readable flat one (default) and a json one, easier to process programmaticaly.

The flat output looks like this:

```
$ php bin/composer licenses --format flat
Root Package:
  composer/composer             1.0.x-dev           MIT
Dependencies:
  justinrainbow/json-schema     1.1.0               none
  phpunit/php-code-coverage     1.2.9               BSD-3-Clause
  phpunit/php-file-iterator     1.3.3               BSD-3-Clause
  phpunit/php-text-template     1.1.4               BSD-3-Clause
  phpunit/php-timer             1.0.4               BSD-3-Clause
  phpunit/php-token-stream      1.1.5               BSD-3-Clause
  phpunit/phpunit               3.7.19              BSD-3-Clause
  phpunit/phpunit-mock-objects  1.2.3               BSD-3-Clause
  seld/jsonlint                 1.1.1               MIT
  symfony/console               dev-master 23d784f  MIT
  symfony/finder                v2.2.1              MIT
  symfony/process               dev-master 9b6fe3a  MIT
  symfony/yaml                  v2.2.1              MIT
```

The json one looks like that:
```
$ php bin/composer licenses --format json
{"name":"composer\/composer","version":"1.0.x-dev","license":["MIT"],"dependencies":{"justinrainbow\/json-schema":{"version":"1.1.0","license":[]},"phpunit\/php-code-coverage":{"version":"1.2.9","license":["BSD-3-Clause"]},"phpunit\/php-file-iterator":{"version":"1.3.3","license":["BSD-3-Clause"]},"phpunit\/php-text-template":{"version":"1.1.4","license":["BSD-3-Clause"]},"phpunit\/php-timer":{"version":"1.0.4","license":["BSD-3-Clause"]},"phpunit\/php-token-stream":{"version":"1.1.5","license":["BSD-3-Clause"]},"phpunit\/phpunit":{"version":"3.7.19","license":["BSD-3-Clause"]},"phpunit\/phpunit-mock-objects":{"version":"1.2.3","license":["BSD-3-Clause"]},"seld\/jsonlint":{"version":"1.1.1","license":["MIT"]},"symfony\/console":{"version":"dev-master 23d784f","license":["MIT"]},"symfony\/finder":{"version":"v2.2.1","license":["MIT"]},"symfony\/process":{"version":"dev-master 9b6fe3a","license":["MIT"]},"symfony\/yaml":{"version":"v2.2.1","license":["MIT"]}}}
```