Link to license text

[cornelius]

The manifest includes an entry for the licenses, which uses short identifier string to indicate the license. It would be great to link to the actual text of the license on the web site.

For this to work we need to standardize the license identifier strings.

We also might need an additional field to store a URL to a text for non-standard licenses. This could be an additional entry in the "urls" section.

[cornelius]

Here is a list of open source licenses: http://opensource.org/licenses

[cornelius]

We could use the license identifiers from SPDX: http://spdx.org/licenses/

[cornelius]

Depends on #75