Added support for "IN" operator of a WHERE clause.

The table update only allowed for "==" operator as in:
  UPDATE mytable SET foo = 'bar' WHERE id = 123;

Now allows for "IN" operator as in:
  UPDATE mytable SET foo = 'bar' WHERE id IN (123, 876);

This required changes to the "_args_to_clause" and "update" functions.
Merge pull request #14 from brettbeaudoin/master

Added support for "IN" operator of a WHERE clause.