Support for file uploading

Support for data objects of type `file`:

```json
{
  "key": "content",
  "value": "tests/integ_tests/d2.json",
  "type": "file"
}
```
## Test
I also created a test file that can be run with: 
`$ bin/newman -c tests/integ_tests/tc4.json -e tests/integ_tests/e2.json`

How ever it currently fails as http://dump.getpostman.com does not support file uploads.  

## File Paths
Currently the `dataObj.value` must be relative to the `cwd` of where you run the newman command.  This isn't as desirable as it being relative to the location of the collection json file, however I was able to locate that resource location. It may make sense to add the location of the json file to the `Globals.requestJSON` object and pull it from there when resolving the path in the `RequestRunner`.