Allow extra fields in composer file

This change adds in support for the `license_uri` and `summary` fields
in the composer file, which match the appropriate locations in the
Magento Connect package.xml.

* `license_uri` is added to the <license> node, in place of the implicit
  use of the Software Package Data Exchange website. This allows for
  links directly to the original website for the license.
* `summary` sets the content for the <summary> node, rather than always
  taking the value from the `description` field.

These changes are optional, so if the fields are omitted then the
package file will be created as it is currently.