package.json: Indicate that the license is the APL 2.0.

Since we are basing our work on the original Enjoy Reading, the original
license is at:

    https://addons.mozilla.org/en-Us/firefox/addon/enjoy-reading/license/1.0.1

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
package.json: Add more specific description of the extension.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>