Update office icons

This PR updates the MS office icons from
![application-msexcel](https://f.cloud.github.com/assets/956847/175262/20f0da2c-7b42-11e2-88fa-655ad42ac442.png) ![application-msword](https://f.cloud.github.com/assets/956847/175263/20f10010-7b42-11e2-99ba-2424c6ddbe41.png) ![application-mspowerpoint](https://f.cloud.github.com/assets/956847/175264/20f4f792-7b42-11e2-8106-7cebac4f47e4.png) ![ms-excel](https://f.cloud.github.com/assets/956847/175265/285a379a-7b42-11e2-903f-3385c9c88eb9.png) ![ms-powerpoint](https://f.cloud.github.com/assets/956847/175266/2a199b84-7b42-11e2-902c-1ac06134a8bc.png)
to 
![application-msexcel](https://f.cloud.github.com/assets/956847/175268/3bdee8e2-7b42-11e2-9224-79c1a7a82682.png) ![application-msword](https://f.cloud.github.com/assets/956847/175270/3be85d1e-7b42-11e2-88c8-609f9b2fc937.png) ![application-mspowerpoint](https://f.cloud.github.com/assets/956847/175269/3be43fb8-7b42-11e2-956d-3ecf012cba32.png) ![ms-excel](https://f.cloud.github.com/assets/956847/175271/4157a14c-7b42-11e2-938c-dce378de32e2.png) ![ms-powerpoint](https://f.cloud.github.com/assets/956847/175272/41565c42-7b42-11e2-8427-8502780597cc.png)

the word icon has a white background. the new icons have a transparent background, look prettier and indicate MSOffice documents more accurately. @jancborchardt @DeepDiver1975 @karlitschek pls have a quick look.