Import UX and Style improvements

Hello,

You will find here many minor improvements for the Contact application. I think it requires extensive functional test.

* Change main font to source sans pro, it deals really better with special chars and font transformation
* Fix margin in several places to make things look proper.
* Name comparison is based on locale comparison to handle special chars like ä, ô, etc.
* Importing is slower (contacts are created one by one) but does not freeze the navigator anymore. A progress indicator is displayed too. The progress indicator is not sexy, I think we could do better later.
* Add a button to export Contact at VCF format.
* Import twice the same name lead to no contact creation.  If two different person have the same name, only one is imported. That's an issue but it's a good trade-off with the fact that two imports no more lead to a lot of duplication.

Before merging please wait I add two automatic tests (import of the same contact twice and vcf export for a single contact).
Github is annoying with that: I commented the first line of "#create-task" but it shows what is before so you misunderstand my comment :-/ gonna be more careful with that next time, sorry!