get_minimal now works from a database

As I have found, the Soundcloud API returns a maximum of 50 results for
most (maybe all) calls. get_minimal.py now reads from the sqlite
database to judge whether to re-collect accounts (based on whether a
certain threshold level has been reached in terms of how much data has
been collected from them and how much remains to be collected).
Unfortunately, this seems to be pointless as tests suggest that you get
the same 50 results every time you execute the same API call (or at
least, that is what was happening last night). To do: add calls to
functions in get_soundcloud_data.py to create a log, add time delay,
etc.

Other change: genre_relationships.py now has a slightly more elegant
way of checking for whether tables are present. This more generalised
function is now also called by get_minimal.py.