Replace telephone numbers in a new group chat with contact names

I replaced the telephone numers in a new group chat with the corresponding contact names or in case of the own number with "Me". I also made some old strings translatable.

I copied the check for the own number from #1505 and put it in isOwnNumber(). Perhaps we could put this method in a place where it could be used from both PR's?

before:
![before](https://cloud.githubusercontent.com/assets/5296073/3379294/30824ffa-fbef-11e3-9b6b-6c739c3cfbdc.png)
after:
![after](https://cloud.githubusercontent.com/assets/5296073/3379296/33605514-fbef-11e3-960f-cc4bcbbdba1e.png)