[FEATURE] Add ability to delete trust line if balance is already 0
[FEATURE] Trustlines: Add ability to delete trust line (WC-1620)

Checks for 3 cases:

1) Trustline has zero balance -> Delete trustline
2) Trustline has balance & there is a market to convert remaining balance to XRP -> Convert remaining balance to XRP to send back to user, delete trust line
3) Trustline has balance & there is no market to convert remaining balance -> Send remaining balance back to the issuer, delete trustline
[FEATURE] Trustline: Fix UI for trustlines - WC-1620
There are now 3 different popups, for each of the three cases.
These changes will provide a clearer UI and messaging to the end users.
[CHORE] Trustlines: Re-implementation of popups - WC-1620
Change setTimeout -> $timeout (angular)
Hard coded text is moved out of JS file
[CHORE] Trustlines: Refactor JS code - WC-1620
[CHORE] Trustlines: Add comments, refactor JS - WC-1620
Trust: if, else, return code style.
[FIX] Trustlines: More dynamic orderbook load detection using $scope.watchCollection - WC-1620
Merge pull request #1211 from ripple/feature/delete_trustline

[FEATURE] Add ability to delete trust line if balance is already 0