Restructure recruiter files

[polvanrijn]

This issue proposes to convert structure of:

dallinger
|_ recruiters
|_ prolific
|_ mturk

To a more general and extendable structure, namely:

dallinger
|_ recruiters
   |_ prolific
       |_ __init__.py
       |_ recruiter.py
       |_ service.py
   |_ mturk
       |_ __init__.py
       |_ messages.py
       |_ qualifications.py
       |_ recruiter.py
       |_ service.py
   |_ __init__py
   |_ recruiter.py
   |_ redis.py
   |_ service.py

This abstraction assumes that most recruiters have a 'service' doing the API calls to the recruiter and the 'recruiter' itself implementing the recruitment logic. How granular this file structure needs to be is open for discussion. A file structure like this would also work:

dallinger
|_ recruiters
   |_ prolific.py
   |_ mturk.py
   |_ __init__py
   |_ recruiter.py
   |_ redis.py
   |_ service.py

[pmcharrison]

Maybe instead of dallinger.recruiters.recruiter we should just put dallinger/recruiters/recruiter.py into dallinger/recruiters/__init__.py and then we can say from dallinger.recruiters import XYZ?