_numpy_from_sample: Improve error reporting

Print a more useful message when wrong type passed to `_numpy_from_sample`.

I ran into this when mistakenly passing a region as the first argument to
`stbt.ocr`.
stbt._set_config: Don't fail if ~/.config/stbt/ doesn't exist

This happens if the user hasn't created `~/.config/stbt/` and hasn't
specified `$STBT_CONFIG_FILE`.
Makefile: Don't use parallel "--no-notice"

Because it isn't available in all versions of GNU parallel (thanks to
Lewis Haley for the heads-up).

I had introduced this in e3aa9fd to avoid verbose output on Travis CI,
so I'll place the workaround in the Travis setup instead.
Merge pull request #189 from drothlis/travis-silence-parallel-citation-notice

Makefile: Don't use parallel "--no-notice"
stbt._find_tessdata_dir: add tessdata directory suffix for Fedora

As tested on Fedora 17, 19 and 20, the `tessdata` directory is
`/usr/share/tesseract/tessdata`, which wasn't being found by
`_find_tessdata_dir`. This commit adds the valid suffix so that, among
other things, the selftests which require this function works.
stbt.py: [pylint] E713 test for membership should be 'not in'

This is bad (apparently):
`not elem in iter`

And this is good:
`elem not in iter`

According to pylint --version
  pylint 1.1.0,
  astroid 1.0.1, common 0.61.0
  Python 2.7.5 (default, Feb 19 2014, 13:47:28)
  [GCC 4.8.2 20131212 (Red Hat 4.8.2-7)]
stbt.py, irnetbox.py: [pylint] E265 block comment should start with '# '

I think we can all agree that this is much better.

According to pylint --version
  pylint 1.1.0,
  astroid 1.0.1, common 0.61.0
  Python 2.7.5 (default, Feb 19 2014, 13:47:28)
  [GCC 4.8.2 20131212 (Red Hat 4.8.2-7)]
stbt.temporary_x_session: use correct check that process has exited

An object returned by Popen can be checked if it is still running, etc.
However the existing check of looking at the `returncode` attribute would
*always* return `None` because it is only *set* when calling `poll` or
`wait`. [1][2]

This commit changes the check for the value returned by `poll`, which is
None if the command has not exited, other the returncode.

This test is failing on my PC because the original command is failing
from the outset, i.e. the x-server is *not* being launched at all.
Without this commit, the test hangs.

[1] Example:
In [18]: vmstat = subprocess.Popen(['vmstat', '1'], stdout=open('/dev/null', 'w'))
In [19]: vmstat.returncode
In [20]: vmstat.poll()
In [21]: vmstat.returncode
In [22]: vmstat.terminate()
In [23]: vmstat.returncode
In [24]: vmstat.poll()
Out[24]: -15
In [25]: vmstat.returncode
Out[25]: -15

[2] https://docs.python.org/2/library/subprocess.html#subprocess.Popen.returncode
Merge branch 'fix-finding-tessdata-dir-and-other-stuff'

* fix-finding-tessdata-dir-and-other-stuff:
  stbt.temporary_x_session: use correct check that process has exited
  stbt.py, irnetbox.py: [pylint] E265 block comment should start with '# '
  stbt.py: [pylint] E713 test for membership should be 'not in'
  stbt._find_tessdata_dir: add tessdata directory suffix for Fedora

Closes #188.