Configurable IPResolutionBolt --> Fetcher stream grouping

This pull request makes three changes:

1. Adding the Maven exec plugin to pom.xml
2. Reorganizing the ``CrawlTopology`` class to separate the topology building from the Main method
3. Adding configurable stream grouping for the ``IPResolutionBolt`` --> ``Fetcher`` tuple stream. Previously, the stream grouping was hardwired to the 'ip' tuple field. With this pull request, the stream grouping should now be configurable to either 'domain' or 'ip,' by setting the property ``fetcher.grouping.scheme`` in ``storm-site.xml``.