Inherit pk weak entities

flask_admin.contrib.sqla.tools.is_inherited_primary_key() now works even if the inherited primary key is itself a foreign key. This leads to a better support of SQLAlchemies *joined table inheritance*

upload images problem

Fixed wrong creation of select2tags components.

Now users can supply data to the attribute "data-select2-tags" to provide
existing tags.

Multiple Primary keys for sqla backend

make upload path auto
fix pic save incomplete
fix 'thumb image return none error'
remove an extra 'return' token
fix dynamic gif thumb error

bugfix: inline form 'new' template - now submits all added fields correc...

Fix "NoneType" has no attr 'get' on subdocumments

in this line  https://github.com/mrjoes/flask-admin/blob/master/flask_admin/contrib/mongoengine/form.py#L44
None causes "NoneType" has no attr 'get'

`contrib.sqla.view.ModelView.action_delete()` now is able to mass-delete models with multiple primary keys. Therefore, two new helper functions has been introduced in `contribg.sqla.tools`:

* `tuple_operator_in()` that fakes the `sqlalchemy.tuple_` operator for engines, that do not support the `tuple_` operator. (like sqlite)
* `get_query_for_ids()` that returns a query, object that contains all objects identified by the `ids`.

Both functions have docstrings with more information.

The logic of the `action_delete()` did not change, as for objects with a single primary key, the codepath did not change at all.