Compatibility with joined-table inheritance

[mush42]

Currently sqlalchemy-mptt tries to set table indexes in the table_args class method. This make sqlalchemy complains when it tries to construct child classes because it couldn't find columns declared in the base class.
I fixed this in my local setup by adding index=True to column definition, instead of defining indexs in a separate class method.

Should I create a PR to fix this?

[uralbash]

@mush42 Yes sure it would be great if you add it

[snitkoff]

@uralbash It would be great if you paid attention to this request #63

[uralbash]

@snitkoff @mush42 Sorry for the wait, I merge your changes