[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 text one (default) and a json one, easier to process programmaticaly.

The text output looks like this:

```
$ php bin/composer licenses --format text
Name: composer/composer
Version: 1.0.x-dev
Licenses: 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"]}}}
```