Automatically add default column families

[Congyuwang]

Now, it's possible not to specify "default" column

Rdict(path, column_families={"default": options, "other_columns": xx})

when reopening Rdict with columns families. The old way is still valid, but a new way is also possible:

Rdict(path, options=options, column_families={"other_columns": xx})

Especially if default Options is used, then just use

Rdict(path, column_families={"other_columns": xx})

[Congyuwang]

This is in fact a bug fix. It might cause compatibility issue with v0.2.x users