Explicit option for local run

[gsakkis]

Currently the only way to specify a local run is by omitting the -hadoop option. However this doesn't work if there is a ~/.dumborc file that specifies a default hadoop option; we have to comment out and uncomment this line in the config for toggling between local and cluster runs. Essentially we need a 'nohadoop' (or 'local') option to override from the command line the hadoop option in the config.

Another quick and dirty workaround would be to denote the local run with a special '-hadoop' value, e.g. " -hadoop '!' ". That's what I ended up implementing in my fork; if you're interested I can upload it and make a pull request.

[klbostee]

This is roughly related to #33.