Improve `VersionManager` to using with custom managers

[tbazadaykin]

Problem

VersionManager is not friendly enough to custom manager, because of:

1. get_queryset always return VersionedQuerySet, as result it's impossible to override query set class throut from_queryset method;
2. get_queryset has some extra code which set querytime property for query set instance. As a result user should copy this two lines - this is not a pythonic way.

Solution

1. Move querytime-code from queryset method into querytime decorator.
2. Some VersionManager methods use VersionedQuerySet methods, so we must ensure that they has deal with VersionedQuerySet or it's subclass. If not - we must warned user about potential problems.

[tbazadaykin]

Btw, why you add Vagrantfile into .gitignore?