Fixed #846, #808. Refactored JOIN logic so it now works with relationship properties instead of tables.

This is partially incompatible change - you will have to update your custom filters. `apply` method now
accepts additional `alias` argument. `alias` should be used to resolve column in query with specific JOIN path.

Long story short, old code which was looking like this:

```python
class MyFilter(SQLABaseFilter):
    def apply(self, query, value):
        return query.filter(self.column == value)
```

Should look like this:

```python
class MyFilter(SQLABaseFilter):
    def apply(self, query, value, alias=None):
        return query.filter(self.get_column(alias) == value)
```