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
Merge pull request #4 from Kjwon15/codecs

Codecs