make check: Enforce pep8 `import` order

PEP8 says: [1]

> Imports should be grouped in the following order:
>
> 1. standard library imports
> 2. related third party imports
> 3. local application/library specific imports
>
> You should put a blank line between each group of imports.

If we're going to follow this convention, let's make the computer check
it to avoid wasting reviewer cycles.

Unfortunately neither the `pep8` tool nor `pylint` have checks to
enforce this. Instead, we use `isort`[2] from PyPI (pip install isort).

If you have trouble getting a file to pass isort's check, run
`isort` (without `--check-only`) on the file to fix it in-place.
To keep this as convenient as possible, don't customise isort's
formatting rules in our pylint.sh.

[1] http://legacy.python.org/dev/peps/pep-0008/#imports
[2] https://pypi.python.org/pypi/isort/
make check: check pep8 import order only if `isort` is installed

`isort` isn't installable with yum/apt-get on Fedora, Debian or Ubuntu;
some developers find it a bit annoying to be forced to use pip, so make
this optional. The important thing is that Travis will check it.
make check: Run pylint before unit & integration tests

Static analysis is faster than our dynamic tests, so if static analysis
is going to fail the build we want to fail as quickly as possible.
make check: Log details when isort fails

isort (3.8.0) passes on my PC, but fails on Travis (isort version
???). We need some diagnostic information.

Note that `isort --diff --check-only` doesn't work; `--check-only` takes
priority and `--diff` is ignored.
Add .isort.cfg to fix false positives on Travis

When run on Travis, `isort` thinks that `binascii` is a 3rd-party
module (it's part of the python standard library), and that `cv2` is a
standard library (it's a third-party library, from `apt-get install
python-opencv`). I don't know why.

I've raised https://github.com/timothycrosley/isort/issues/157
Merge pull request #154 from drothlis/make-check--import-order

make check: Enforce pep8 `import` order
OCR: Add script for measuring OCR effectiveness

validate-ocr.py can be run on a corpus of test images reporting how good a
job stbt has done of reading the text.  Thus it can be used to measure
improvements to the OCR algorithm and, more importantly, detect any
regressions introduced.

This tool is designed such that it can be run on corpuses outside the
stb-tester git tree to allow corpuses containing screen captures from many
set-top boxes without bloating the main stb-tester repo or risking
upsetting the owners of the creators of the various set-top box UIs.

It is not currently run in-tree as a part of make check.

Thanks to david@rothlis.net and lewis.haley@youview.com for review and
fixups.