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
I forgot add 'Boolean' to __all__ and valid check on 'Rfc822'
Content element
add test code for Rfc822
raise error when tzinfo was not valid or none
remove double check isinstance
Entry element
Python 3 failed to invoke pep8
Merge branch 'codecs' of git://github.com/Kjwon15/libearth
Style adjustments
Unnecessary to check None values
Prefer abstract base class over concrete int type

Why we should prefer numbers.Integeral which is
an abstract base class over int which is a
concrete one?  There could be several reasons for
this:

1. Portability: Python 2.x have actually two
   concrete integral types: int and long.
   We will fail when try isinstance(0L, int).

2. Flexibility: There might be custom types that
   behave like integer, actually subclasses
   numbers.Integral.  These also should be dealt.
Docstring adjustments
Merge remote-tracking branch 'upstream/master' into feedlist
replace 'Boolean', 'Enum' in alphabetical order