Add option to change row class

The row class returined by iterating over rows.Table could be a dict a collections.namedtuple or even a customized class representing that data. We need to provide an way to the user to specify this class. Actually, preferably the user should pass a metaclass (think of collections.namedtuple: when you call it the object returned is a Python class).
We may create an interface for it (instead of just passing dict for example, we may need to create a RowDict class that does some things).

Another option: use the attrs library.

Possible API: add row_class parameter to import functions, like in:

for book in rows.import_from_csv(csv_path, row_class=dict):
    print(book["title"])

Related to #304.

Note: check if we can integrate this feature with scrapy so it'll easier to parse data using rows in a scrapy project.

Start by inspecting rows.Table and the import functions, especially the rows.import_from_csv API, to understand where row objects are created. Define and test the requested row_class behavior for dict, namedtuple-style factories, and custom classes, while checking whether the design can support the Scrapy use case and related issue #304.