Bug 843505 - Define a generic component for importing contacts from diff...

...erent sources

First version: Known issues:

* Facebook needs to refactor to adapt to the new generic structure, that will be another bug
* L10N needs refactoring, that will be done under other bug or the FB one. 
* It includes a very first version for importing Hotmail contacts from FTE, but that needs to be refined as per UX and product input and later it would need a refactoring in FTE code (probably another bug). 
* Needs a few unit tests (ongoing) 

This bug should be landing after 843507 (Windows Live Connector). 

/cc @arcturus @crdlc