Add feature retry attempts to work around xcode 5 stability issues

This pull request is to enable zucchini to continue to function as expected whilst apple fix their issues with xcode 5 / instruments freezing and hanging randomly when executing ui automation tests.

### Current problem
When running UIAutomation tests the `instruments` command will start up but the app will hang \ freeze on the splash screen in the simulator.  It will stay like this until manual intervention to kill the job and start it again.

A quick google search provides that others since upgrading to xcode 5 etc have been experiencing this issue as well.
### Provided solution
There are 2 parts to this pull request, one is to specify the feature execution timeout in seconds.  This is the max length of time a feature should be allowed to run.  If it exceeds this timeout then it will kill the `instruments` process and depending on the number of retries provided in the config.yaml will kick off the feature again.

**config.yaml snip**
```
  feature:
    timeout: 90
    retry_attempts: 3
```

**Please note no tests have been added for this functionality, but am keen to get feedback on this solution**