[FIX] Path finding stopped working with gateway account (RT-2218, RT-2215)

* Specifying every currency/issuer pairs with positive amounts only while
  doing path-finding.
* Allow issue currency when the currency has both positive and negative
  components.
* Fix path displayed as n/a.
  (related to "Selected currency should be the first option")
* Fix a bug when exchanging currencies without full names.
Merge pull request #1514 from orzFly/fix/rt-2218

[FIX] Path finding stopped working with gateway account (RT-2218, RT-2215)