Use sh instead of manual subprocess

[clybob]

Use library sh instead of call manual subprocess.
Documentation sh library: http://amoffat.github.com/sh/

[clybob]

The output generated by sh is bad, because Pep8 and Pyflakes don't returns exit code. It's necessary find to solve this problem before apply sh. The output generated:

-> sh.pep8('.', _out=process_output)
./code_quality_tools/tests/fixtures/pep8.py:2:1: E302 expected 2 blank lines, found 1

./code_quality_tools/tests/fixtures/pep8.py:5:1: E302 expected 2 blank lines, found 1

./code_quality_tools/tests/fixtures/pep8.py:6:17: W292 no newline at end of file

Traceback (most recent call last):
File "", line 1, in
File "/Users/romulo.tavares/.virtualenvs/dashboard/lib/python2.7/site-packages/sh.py", line 354, in repr
try: return str(self)
File "/Users/romulo.tavares/.virtualenvs/dashboard/lib/python2.7/site-packages/sh.py", line 333, in str
else: return unicode(self).encode(self.call_args["encoding"])
File "/Users/romulo.tavares/.virtualenvs/dashboard/lib/python2.7/site-packages/sh.py", line 337, in unicode
if self.stdout: return self.stdout.decode(self.call_args["encoding"])
File "/Users/romulo.tavares/.virtualenvs/dashboard/lib/python2.7/site-packages/sh.py", line 280, in stdout
self.wait()
File "/Users/romulo.tavares/.virtualenvs/dashboard/lib/python2.7/site-packages/sh.py", line 266, in wait
self._handle_exit_code(self.process.wait())
File "/Users/romulo.tavares/.virtualenvs/dashboard/lib/python2.7/site-packages/sh.py", line 275, in _handle_exit_code
self.process.stderr
sh.ErrorReturnCode_1:

RAN: '/Users/romulo.tavares/.virtualenvs/dashboard/bin/pep8 .'

STDOUT:
./code_quality_tools/tests/fixtures/pep8.py:2:1: E302 expected 2 blank lines, found 1
./code_quality_tools/tests/fixtures/pep8.py:5:1: E302 expected 2 blank lines, found 1
./code_quality_tools/tests/fixtures/pep8.py:6:17: W292 no newline at end of file

STDERR: