make company model swappable

[imposeren]

Company model should be swappable. This should fix:
#25 #28

Also when custom model is used TestRunner is unable to serialize data for openninghours.Company:

    return self.cursor.execute(sql, params)
django.db.utils.ProgrammingError: relation "openinghours_company" does not exist
LINE 1: ...mpany"."slug", "openinghours_company"."logo" FROM "openingho...

This MR should fix this too

[coveralls]

Coverage increased (+0.05%) to 85.531% when pulling 00587e1 on imposeren:use-swappable-model into 0a80b43 on arteria:master.

[SirEdvin]

@arteria, can you please merge it?

[imposeren]

@jvamvas, @walterrenner, can anyone from arteria please merge this and release new version on PyPI?