Refactor Runner handling

[zstumgoren]

Related to #91 and #61

Refactor handling of runners to support different call signatures of Site classes (Odyssey, Oscn, Wiscourts) and their search methods. Options:

• Update cli.py to find and invoke platform-specific runners (e.g. oscn/runner.py)
• Registry of site types and their related runners (seems wasteful to add this in sites_meta.csv, so probably should go in cli.py)

Tasks

Refactor Runner handling

• Refactor cli.py to invoke runners from inside platform-specific modules (e.g. oscn/runner.py,odyssey_site/runner.py) in orde
• Add oscn/runner.py
• Add cli integration test for oscn