Use xing_api gem to load the xing user profile

### what changed
* use xing_api gem to facilitate loading xing profile
* add rspec test suite and test touched code
* save oauth token on the user model
* add a profile_loaded field to the user model, to track which users have up-to-date data
* remove unused /test directory
* add config for travis ci

### comments
* travis ci is currently testing fork. TODO update README.md
* profile_loaded field is not used to filter out outdated profiles yet