Fix logging output directory

When main.py was moved into performanceplatform.collector the logging
path was not changed. This should expect and output to a directory in
the root of the project called log.
Call pp-collector rather than main.py

It is not polite to call down into nested packages when we expose a
named script you can run. This changes the outputted cronjobs to call
this script rather than main.py
Merge pull request #11 from alphagov/deployment-fixes

Deployment fixes
Add `json_fields` parameter to set_up_logging

This will allow the main function to add extra fields to JSON log
messages, for example to pass through command-line arguments.

See https://www.pivotaltracker.com/story/show/70748012
Add extra fields to JSON log

Add the data group/type and a stringified version of the actual query
we're running.

The extra fields in the JSON log message look like this:
```
{
  ...
  "@data_group" : "govuk",
  "@data_group_data_type" : "govuk/visitors",
  "@data_type" : "govuk",
  "@query" : "id=ga:56580952 metrics=[u'visitors']"
}
```

https://www.pivotaltracker.com/story/show/70748012

[#70748012]