fix icons license

This fixes #10 and https://github.com/sebsauvage/Shaarli/issues/8.

How it looks:
![linklist](https://cloud.githubusercontent.com/assets/3950390/3778219/b46fb274-196f-11e4-81ab-0243044dc037.png)
![private](https://cloud.githubusercontent.com/assets/3950390/3778222/b92ee2ee-196f-11e4-9d55-10813768dced.png)
![daily](https://cloud.githubusercontent.com/assets/3950390/3778223/bc408d16-196f-11e4-8e8a-f854ec570833.png)

it's now in [COPYING](https://github.com/nodiscc/Shaarli/blob/f27621eb4117a35e918d5ca9c0891be2c3e73ed7/COPYING), with a reminder [at the end of README.md](https://github.com/nodiscc/Shaarli/commit/f27621eb4117a35e918d5ca9c0891be2c3e73ed7#diff-04c6e90faac2675aa89e2176d2eec7d8R49). Or i could place all copyright notices in the README but it would quickly become a mess.