Show invitee username

![screenshot from 2014-12-10 14 36 39](https://cloud.githubusercontent.com/assets/464938/5376715/fcfbe8a2-8079-11e4-8123-47a75219c440.png)
Merge PR in ix first: stellar/ix#424
Closes: #1005
Why hide on ```ng-if="invite.inviteeId"``` when the content is in ```invite.inviteeUsername```?

Although this may work, it is not semantically correct. In the future the relationship between these two variable could change and break this feature. The best way to handle ```invite.inviteeUsername``` being undefined is to test if ```invite.inviteeUsername``` is undefined.