stbt-run: Cleanup: Remove unused import

Unused since 78a50d12.
Cleanup: Only hide argv from the "gst" python module

This makes it clearer that ArgvHider exists to work around a bug in that
specific module. See commit 1bd421b4 for details.
stbt run/record: Hide 'pygobject_register_sinkfunc is deprecated' message

This message was shown every time you ran `stbt run` or `stbt record`.

The `gst` module doesn't use python's `warnings` module; in fact the
message is probably coming from C code because re-assigning python's
sys.stderr to os.devnull doesn't help. For details see this blog post:
http://blog.greghaskins.com/python/1/forcibly-redirect-stdout-stderr-from-extension-modules/

If an exception is thrown from the "import gst" statement (e.g. you
don't have that python library installed) then the exception escapes the
`StdErrHider` context manager, so the exception message *is* printed.
README: Cleanup format of example config file

To my preferred formatting. This shows the reader that indentation is
not necessary, and that spaces are allowed around the "=".
make check-integrationtests: Use ximagesink instead of user's sink_pipeline

On my OS X system, where my user's stbt.conf says to use `glimagesink`,
when I run the tests I get interrupted with a system dialog saying:

    The application "Python" unexpectedly quit while trying to restore
    its windows. Do you want to try to restore its windows again?

Using ximagesink avoids the above problem.

Note that the tests that use `gst-launch` directly (instead of `stbt
run`) *already* use ximagesink (see e.g. tests/test-templatematch.sh)
so this is also more consistent, and there are no users (of these tests)
without X11 to worry about.
stbt.load_defaults: Cleanup: ConfigParser `add_section` isn't necessary

I think it's only necessary for writing config files, not for reading
from them.
stbt-completion: Cleanup: Remove initial "#!bash" line

It isn't actually a valid interpreter directive, because it doesn't have
an absolute path. It *shouldn't* be an interpreter directive because the
script is intended to be sourced into your shell, not run as a
subprocess; but I think it could be confusing to some people because it
looks *almost* like an interpreter directive.

It did serve one purpose: Telling editors like Emacs what type of file
this is, for syntax highlighting etc. The new "-*- shell-script -*-"
has the same effect.