avocado.core.output: Add missing docstrings

So we get even more complete API documentation.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.utils.process: Improving docstrings

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.result: Docstring fixes and improvements

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.sysinfo: Improve docstrings

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.utils.misc: Improve docstrings

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.utils.memory: Improve docstrings

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
docs: Fix install instructions in GetStartedGuide

The commands described did not work under some circumstances,
so change them to safer versions.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
.travis.yml: Add some functional testing of avocado

Let's actually execute avocado in scenarios where we
know what the produced outcome is:

* Execute a list of tests that pass, expected rc == 0
* Execute a list of tests where some fail, expected rc == 1

Although very simple, it's already a good start of
functional testing of the code during each travis job.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>