Added switch to disable accumulators + bugfix + api change

This includes

- bugfix for @AHeise (https://github.com/stratosphere/stratosphere/issues/349)
- small change in the API as proposed by @rmetzger (https://github.com/stratosphere/stratosphere/issues/119) 
- the introduction of a switch to disable accumulators per job.
- two more default accumulators for averages and Min/Max. They are not perfect yet, so I only added them to the AccumulatorITCase so you can make them default if you want.

Regarding the switch

- I found that it's better to store the disableAccumulator in the job configuration object (and not in each TaskConfig). This way we have less overhead and I can easily read the property in the JobClient to avoid the additional RPC call if accumulators are disabled.
- Let me know if you propose other cli-options. I could not test it, because I always get the following error

        bin/stratosphere
        Exception in thread "main" java.lang.NoClassDefFoundError: eu/stratosphere/client/CliFrontend
        Caused by: java.lang.ClassNotFoundException: eu.stratosphere.client.CliFrontend