Start working on crawler

In order to send pull request
Add missing docs

As Minhee mentioned.
Make Crawler class and basic methods
Modify auto_discovery method to suit test.
Change string's double quote to single quote.
Modify get_document_type() code.

Read the document, not f.info().type, and identify the document's type.
In addition, UnidentifiedDocument class is created to raise Exception
when method cannot identify the document's type.