Start working on crawler

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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.
Add class attributes and methods.

__init__(), get_document_type, reset method. And methods normally return to
set class attributes.
Codec
Common codecs
Enum codec
Add missing name to __all__
Markup tag cleaner
Text construct
Person construct
Link construct
Category construct
added Integer, Rfc822 codec

Conflicts:
	libearth/codecs.py
change arguments of Boolean codec

Conflicts:
	libearth/codecs.py
raise when Boolean.encoding or decoding

Conflicts:
	libearth/codecs.py
text_type -> string_type. add test codes

Conflicts:
	tests/codecs_test.py
start documentation codecs.py