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.
Start working on crawler

In order to send pull request
Make Crawler class and basic methods
Modify auto_discovery method to suit test.
Change string's double quote to single quote.
Add class attributes and methods.

__init__(), get_document_type, reset method. And methods normally return to
set class attributes.
Disassemble class into functions.

In addition, add crawler test and modify auto_discovery function
Make parser.py and parser_test.py.
Parsing required tags in Feed Document.
Remove unused Errors.
Parsing feed element except entry element.
First commit for feed.py
basic functions to feed list
fix function args, add test codes
fix docstring for valid Sphinx syntax
add feed.rst
change data type, adjust code. and remove `__repr__` function