Revert "Remove extra/stbt_checker.py"

This reverts commit 97eada336b11e9730506cef2452d03622f967080.

I had reverted this because it doesn't work with pylint 1.0. However,
not much *does* work with pylint 1.0†, and this tool is still useful
for those of us with pylint 0.x still installed.

† https://bitbucket.org/logilab/pylint/issue/47/
Move extra/stbt_checker.py to stbt_pylint_plugin.py

This is in preparation for creating the "stbt lint" tool.
extra/stb-tester.spec: Cleanup: Sort "Requires" lines alphabetically
stbt lint: Runs pylint (with custom plugin) over stbt scripts

Currently the only additional check done (additional to pylint's
built-in checks) is E7001: Image paths given to
`stbt.wait_for_match` (and similar functions) must exist on disk.

To configure pylint's behaviour:

* Arguments to `stbt lint` will be passed on to pylint.
* Modify ~/.pylintrc, /etc/pylintrc, or set the PYLINTRC environment
  variable, or pass `--rcfile=<file>`.
* In your stbt scripts, inline comments like `# pylint: disable=E7001`
  will disable the specified checker for a single line, function, or
  class.

To use this in `make check` or similar automated checking of your
repository of stbt scripts, use something like this:

    shopt -s globstar
    stbt lint --errors-only **/*.py

Note that (the existing script) extra/pylint.sh is a script used by
stb-tester *developers* to ensure that the core stb-tester code complies
with our coding standards. On the other hand the new `stbt lint` tool is
for stb-tester *users* to run over their test scripts.

`stbt lint` doesn't work with pylint 1.0. See 97eada33 and 65a38380 for
details.
Merge branch 'stbt-lint'

* stbt-lint:
  stbt lint: Runs pylint (with custom plugin) over stbt scripts
  extra/stb-tester.spec: Cleanup: Sort "Requires" lines alphabetically
  Move extra/stbt_checker.py to stbt_pylint_plugin.py
  Revert "Remove extra/stbt_checker.py"