Fix opacity on favicon

Fixes the opacity on the favicon so it doesnt look awkward on non
white browser chrome

Original patch by Benjamin Anderson

Closes COUCHDB-1133

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Before:
![bildschirmfoto 2014-08-11 um 00 17 58](https://cloud.githubusercontent.com/assets/298166/3870741/1f6f4a94-20dc-11e4-8721-d5f5f0bf4080.png)

After:
![bildschirmfoto 2014-08-10 um 22 34 03](https://cloud.githubusercontent.com/assets/298166/3870735/8a9e6ff8-20db-11e4-9836-3602f906e191.png)