Add basic online tests

Issue #3 was only discovered by running a live test against an
online Gerrit instance in my non-default Python version. This raised
issue 4 (Unit tests are not sufficient).

The new tests in test_online.py will now perform some basic online
sanity checks if the environment variable GSSH_TEST_INSTANCE is set
with the netloc for an active Gerrit instance.

Tests are not exhaustive and take time to complete, so they are
normally skipped. For now, they connect to the site; check the
returned version string; grab a list of all projects; and query
for the first three open code reviews. The list can be extended
as needed in the future, but for now that covers many of the areas
which have produced problems.

Online tests can be run as follows:

    GSSH_TEST_INSTANCE='example.gerrit.com' py.test

or

    GSSH_TEST_INSTANCE='example.gerrit.com' tox
Fix bug in ssh implementation

The online checks actually discovered a typo in the ssh client code
which surfaced when disconnecting.

Normally, the ssh client is mocked out during unit tests.
Add new test and clean targets to Makefile

Test targets were added to run py.test and tox with the test instance
variable set to review.openstack.org, so that online tests can be run

   make test-online quickly runs py.test
   make test-all-online runs tox including online checks

Expanded the range of clean targets to include make clean-dist,
which removes all the tox virtual environments and any .egg files
found in the tree.

make targets clean and clean-pyc now also remove all __pycache__
directories. make clean and clean-build clean out the docs and coverage
output files.
Prepare for release 0.1.3

Update documentation and meta data.

Fixes #4