Fix for spark.task.maxFailures not enforced correctly.
Fix tests.
Merge pull request #245 from gregakespret/task-maxfailures-fix

Fix for spark.task.maxFailures not enforced correctly.

Docs at http://spark.incubator.apache.org/docs/latest/configuration.html say:

```
spark.task.maxFailures

Number of individual task failures before giving up on the job. Should be greater than or equal to 1. Number of allowed retries = this value - 1.
```

Previous implementation worked incorrectly. When for example `spark.task.maxFailures` was set to 1, the job was aborted only after the second task failure, not after the first one.