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