README: Add Travis CI build status icon

So that the build status will be visible in the GitHub web interface
at https://github.com/drothlis/stb-tester (which displays the README
in the main project page).

travis-ci.org is a free Continuous Integration service -- see commit
b08660e5 for details.

When we generate the stbt man page from this README, we have to remove
this image, because man pages can't have images and we'd end up with
an ugly placeholder instead.
Disable annoying pylint warnings

Disable the following pylint warnings:

* All "C" conventions, because some of them (like "missing docstring")
  are too draconic. The exception is C0301 (line too long) which we do
  want to enforce. In fact, enabling the "line too long" check was the
  initial motivation for enabling pylint warnings in 1276c4e3.

* All "R" refactor warnings, because I don't believe code quality can be
  automatically measured by things like "a class must have at least two
  public methods".

I didn't find any way of wrapping the "disable" line in pylint.conf: A
trailing "\" doesn't work, nor does specifying "disable" multiple times
on different lines.

Use "pylint --list-msgs" for a description of all possible warnings.
Use "pylint --help-msg C0301" for a description of a particular warning.

Note that "pylint: enable=C0111" doesn't actually work now that C0111 is
disabled in pylint.conf, so I've removed those comments; these public
functions all have docstrings already.
Remove some pylint inline comments

Instead of telling pylint to ignore W0613 (unused argument), name
the argument with a leading underscore; this also causes pylint to
ignore it.

And fix string escapes to comply with the python spec, instead of
ignoring W1401 (anomalous backslash in string).
tests: Make sure to use the right configuration file

Some of the tests use configuration parameters from stb-tester/stbt.conf.
If the user has STBT_SYSTEM_CONFIG set to any other value then those
tests fail.

'run-tests.sh' makes sure that STBT_SYSTEM_CONFIG is not set to get the
parameters from the right config file.
tests: Remove unused parameters from stbt.conf

tests/stbt.conf has a set of templatematch parameters in its [run] section.
These parameters are never used because stbt gets these parameters from
the [global] section of stb-tester/stbt.conf.

These parameters are removed to avoid confusion.