Include font license url in Developer API JSON response

Since the font file urls can be accessed via the Developer API, and the font's license needs to be distributed with those files, it would be helpful to include the url to the font's license ".txt" file in the Developer API JSON response.

The url could be placed in the "files" field:
` "files": { `
` "regular": "http://themes.googleusercontent.com/static/fonts/antic/v4/hEa8XCNM7tXGzD0Uk0AipA.ttf", `
` "license": "http:///OFL.txt" `
` }`

or a new "license" field could be added that includes information on what license the font has been released under along with the url to the file:
` "license": { `
` "type": "OFL/UFL/APACHE", `
` "file": "http:///.txt" `
`}`