Adding PEP8 only lint rule to tox and Travis.

On my machine, with your branch, I'm seeing a bunch of failures when not limiting 'pep8 gcloud':


    $ tox -e lint
    GLOB sdist-make: /home/tseaver/projects/agendaless/Google/src/gcloud-python/setup.py
    lint inst-nodeps: /home/tseaver/projects/agendaless/Google/src/gcloud-python/.tox/dist/gcloud-0.02.2.zip
    lint runtests: PYTHONHASHSEED='4291643478'
    lint runtests: commands[0] | pep8
    directory .
    checking ./setup.py
    directory ./gcloud.egg-info
    directory ./cryptography-0.6-py2.7-linux-x86_64.egg
    directory ./cryptography-0.6-py2.7-linux-x86_64.egg/cryptography
    checking ./cryptography-0.6-py2.7-linux-x86_64.egg/cryptography/_Cryptography_cffi_4ed9e37dx4000d087.py
    ./cryptography-0.6-py2.7-linux-x86_64.egg/cryptography/_Cryptography_cffi_4ed9e37dx4000d087.py:2:4: E111 indentation is not a multiple of four
    ...
    checking ./cryptography-0.6-py2.7-linux-x86_64.egg/cryptography/hazmat/backends/openssl/hmac.py
    checking ./cryptography-0.6-py2.7-linux-x86_64.egg/cryptography/hazmat/backends/openssl/rsa.py
    directory ./cryptography-0.6-py2.7-linux-x86_64.egg/EGG-INFO
    directory ./httplib2-0.9-py2.7.egg
    directory ./httplib2-0.9-py2.7.egg/EGG-INFO
    directory ./httplib2-0.9-py2.7.egg/httplib2
    checking ./httplib2-0.9-py2.7.egg/httplib2/__init__.py
    ./httplib2-0.9-py2.7.egg/httplib2/__init__.py:68:15: E261 at least two spaces before inline comment
    ./httplib2-0.9-py2.7.egg/httplib2/__init__.py:70:5: E301 expected 1 blank line, found 0
    ...
    ./httplib2-0.9-py2.7.egg/httplib2/socks.py:435:30: E711 comparison to None should be 'if cond is None:'
    directory ./six-1.8.0-py2.7.egg
    checking ./six-1.8.0-py2.7.egg/six.py
    ./six-1.8.0-py2.7.egg/six.py:12:80: E501 line too long (80 > 79 characters)
    ...
    ./six-1.8.0-py2.7.egg/six.py:757:5: E129 visually indented line with same indent as next logical line
    directory ./six-1.8.0-py2.7.egg/EGG-INFO
    directory ./gcloud
    checking ./gcloud/__init__.py
    checking ./gcloud/connection.py
    checking ./gcloud/credentials.py
    checking ./gcloud/demo.py
    checking ./gcloud/test_connection.py
    checking ./gcloud/test_credentials.py
    directory ./gcloud/storage
    checking ./gcloud/storage/__init__.py
    checking ./gcloud/storage/acl.py
    checking ./gcloud/storage/bucket.py
    checking ./gcloud/storage/connection.py
    checking ./gcloud/storage/exceptions.py
    checking ./gcloud/storage/iterator.py
    checking ./gcloud/storage/key.py
    checking ./gcloud/storage/test___init__.py
    checking ./gcloud/storage/test_acl.py
    checking ./gcloud/storage/test_bucket.py
    checking ./gcloud/storage/test_connection.py
    checking ./gcloud/storage/test_exceptions.py
    checking ./gcloud/storage/test_iterator.py
    checking ./gcloud/storage/test_key.py
    directory ./gcloud/storage/demo
    checking ./gcloud/storage/demo/__init__.py
    checking ./gcloud/storage/demo/__main__.py
    checking ./gcloud/storage/demo/demo.py
    directory ./gcloud/datastore
    checking ./gcloud/datastore/__init__.py
    checking ./gcloud/datastore/connection.py
    checking ./gcloud/datastore/dataset.py
    checking ./gcloud/datastore/entity.py
    checking ./gcloud/datastore/helpers.py
    checking ./gcloud/datastore/key.py
    checking ./gcloud/datastore/query.py
    checking ./gcloud/datastore/test___init__.py
    checking ./gcloud/datastore/test_connection.py
    checking ./gcloud/datastore/test_dataset.py
    checking ./gcloud/datastore/test_entity.py
    checking ./gcloud/datastore/test_helpers.py
    checking ./gcloud/datastore/test_key.py
    checking ./gcloud/datastore/test_query.py
    checking ./gcloud/datastore/test_transaction.py
    checking ./gcloud/datastore/transaction.py
    directory ./gcloud/datastore/demo
    checking ./gcloud/datastore/demo/__init__.py
    checking ./gcloud/datastore/demo/__main__.py
    checking ./gcloud/datastore/demo/demo.py
    directory ./cffi-0.8.6-py2.7-linux-x86_64.egg
    checking ./cffi-0.8.6-py2.7-linux-x86_64.egg/_cffi_backend.py
    ./cffi-0.8.6-py2.7-linux-x86_64.egg/_cffi_backend.py:2:4: E111 indentation is not a multiple of four
    ...
    ./cffi-0.8.6-py2.7-linux-x86_64.egg/cffi/verifier.py:239:1: E302 expected 2 blank lines, found 1
    directory ./docs
    directory ./docs/_components
    directory ./docs/_templates
    directory ./docs/_static
    directory ./docs/_build
    directory ./docs/_build/doctrees
    directory ./docs/_build/html
    directory ./docs/_build/html/_sources
    directory ./docs/_build/html/_static
    directory ./docs/_build/html/_images
    directory ./docs/_build/html/_modules
    directory ./docs/_build/html/_modules/gcloud
    directory ./docs/_build/html/_modules/gcloud/storage
    directory ./docs/_build/html/_modules/gcloud/datastore
    directory ./docs/examples
    directory ./docs/examples/expenses
    directory ./docs/examples/expenses/expenses.egg-info
    directory ./docs/examples/expenses/expenses
    directory ./docs/examples/expenses/expenses/scripts
    ERROR: InvocationError: '/home/tseaver/projects/agendaless/Google/src/gcloud-python/.tox/lint/bin/pep8'
    ___________________________________ summary ____________________________________
    ERROR:   lint: commands failed