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.
Fix multiple contents of same type

This adds a fix for a problem that would occur when using multiple
files or directories of the same "type" - each additional content
definition would override the previous one. This was corrected by using
numbered keys into the content array, rather than a string key based on
the type.