follow PEP8 coding style and use pytest/tox for testing

I improved the coding style and fixed some faults for doctest. Improving these changes makes developers easy to maintain and extend feature.

About testing, you can run as follows.

(pyxshell)$ pip install tox pytest pytest-pep8
(pyxshell)$ py.test -v --pep8 --doctest-modules src/pyxshell
=================== test session starts ===================
platform darwin -- Python 2.7.3 -- pytest-2.3.4 -- /Users/t2y/.virtualenvs/pyxshell/bin/python
cachedir: /Users/t2y/work/repo/github/pyxshell/.cache
plugins: cache, pep8
collected 6 items 

src/pyxshell/__init__.py:0: PEP8-check PASSED
src/pyxshell/__init__.py: [doctest] PASSED
src/pyxshell/common.py:0: PEP8-check PASSED
src/pyxshell/common.py: [doctest] PASSED
src/pyxshell/pipeline.py:0: PEP8-check PASSED
src/pyxshell/pipeline.py: [doctest] PASSED

(pyxshell)$ tox 
... (snip)
py26: commands succeeded
py27: commands succeeded
congratulations :)