Feature: automate database downloading

changes:

• feature: Automatically download databases that are specified in the job-config. On first run of prophane, run prophane init {DB_DIR} where DB_DIR is an empty or non existant directory. To execute prophane, run prophane run {path-to-job-config}.
• breaking change: change command line interface (see above):
  • prophane -> prophane.py run
  • prophane --list-dbs -> prophane.py list-dbs
  • prophane --list-styles -> prophane.py list-styles
• bump the db_schema version to 5
• task files now include the database version number instead of the md5sum: tasks/{annot_type}_annot_by_{tool}_on_{db_type}.v{db_version}.task{taskid}.{ext}

associated issues

Closes #66 (closed), #41 (closed), #6 (closed)