NPE when no tags on instance

When no tags exists on other running instances and if we try to filter by tag, we get the following error:

```
[2014-05-19 16:17:37,377][DEBUG][discovery.gce            ] [Theresa Cassidy] start building nodes list using GCE API
[2014-05-19 16:17:37,378][INFO ][cloud.gce                ] [Theresa Cassidy] starting GCE discovery service
[2014-05-19 16:17:37,592][TRACE][discovery.gce            ] [Theresa Cassidy] gce instance hadoop1 with status RUNNING found.
[2014-05-19 16:17:37,597][TRACE][discovery.gce            ] [Theresa Cassidy] start filtering instance hadoop1 with tags [elasticsearch, dev].
[2014-05-19 16:17:37,597][TRACE][discovery.gce            ] [Theresa Cassidy] comparing instance tags null with tags filter [elasticsearch, dev].
[2014-05-19 16:17:37,597][WARN ][discovery.gce            ] [Theresa Cassidy] Exception caught during discovery java.lang.NullPointerException : null
[2014-05-19 16:17:37,597][TRACE][discovery.gce            ] [Theresa Cassidy] Exception caught during discovery
java.lang.NullPointerException
    at org.elasticsearch.discovery.gce.GceUnicastHostsProvider.buildDynamicNodes(GceUnicastHostsProvider.java:157)
    at org.elasticsearch.discovery.zen.ping.unicast.UnicastZenPing.sendPings(UnicastZenPing.java:245)
    at org.elasticsearch.discovery.zen.ping.unicast.UnicastZenPing$2.run(UnicastZenPing.java:176)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    at java.lang.Thread.run(Thread.java:744)
[2014-05-19 16:17:37,598][DEBUG][discovery.gce            ] [Theresa Cassidy] 0 node(s) added
[2014-05-19 16:17:37,598][DEBUG][discovery.gce            ] [Theresa Cassidy] using dynamic discovery nodes []
```

Closes #22.
Update instructions for GCE

Some tools have changed on GCE platform.
We need to update the documentation.

Closes #23.