make check: Enable pylint warnings and style checks

This change (1) removes the `--errors-only` flag from the pylint checker
so that now pylint will warn about refactors, conventions, and warnings
and not just errors, and (2) applies all of the pylint recommendations
to the affected python modules.

Some sensible global pylint exceptions as well as module-specific
exceptions are added in `extra/pylint.sh`. In acceptable situations,
inline pylint "disable" codes are added in the code.

A notable change for compliance is that the stbt.py `display` and
`control` variables are now module-private variables `_display` and
`_control`.

[Committer's note: The motivation for this change is that, while we want
to enforce an 80-character line length, there are some specific cases
(tabular data-driven code, usually) where longer lines make for more
readable code. The `pep8` tool doesn't allow you to selectively disable
errors for a specific line, so this commit disables pep8's line length
check, and enables pylint's equivalent check (along with pylint's other
style checks, warnings, refactoring suggestions, etc).

If you're wondering what a specific pylint error code means in the
inline pylint directives like "# pylint: disable=R0902", use
`pylint --help-msg=R0902`.
]
Fix `make check-pylint` errors found by pep8 v1.4.2

...introduced in the previous commit, which would have been tested only
with pep8 1.0.1.

pep8 1.4.2 performs more rigorous checks than Fedora 17's pep8 1.0.1.

This fixes the following violations:

* E502 the backslash is redundant between brackets
  (stbt.py:350, irnetbox.py:297)
* E128 continuation line under-indented for visual indent
  (stbt.py:351, irnetbox.py:298)
Fix false-pass of test_gstreamer_core_elements

If the `gst-launch` command runs successfully but the pipeline never
enters the PLAYING state, the test used to pass.

This happened on OS X if I didn't have an X11 server running. This
particular test would pass, but subsequent tests would fail. The point
of this test is to catch such environmental issues.