Allow nats to choose a random port when given port -1.

* In order to allow nats to resolve a randomly-chosen port, we want to
  sometimes give net.Listen a port of 0. However, a port of 0 in the
  config resolve to a constant port. In order to avoid behavioral
  changes, we define '-1' to be the default "choose random port"
  number.
* Resolve ports can be retrievd using the server's Addr() method.
PR feedback (nits)
Write back resolved port to options.
Merge pull request #32 from apcera/allow-external-listener

Allow nats to choose a random port when given port -1