Better support for .dockerignore

[aanand]

Builds on @mikexstudios' work in #604, with fixes for Python 2.6 and some refactoring.

Still waiting on version 0.3.4 of pathspec, though - any word on that, @cpburnz?

[aanand]

Test setup is failing for Python 3.2 on this line, which is odd.

  Cloning git://github.com/cpburnz/python-path-specification.git to ./.tox/py32/src/pathspec
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 20, in <module>
      File "/home/travis/build/docker/docker-py/.tox/py32/src/pathspec/setup.py", line 6, in <module>
        from pathspec import __author__, __email__, __license__, __project__, __version__
      File "pathspec/__init__.py", line 11
        __copyright__ = u"Copyright © 2013-2015 Caleb P. Burns"
                                                               ^
    SyntaxError: invalid syntax

[cpburnz]

@aanand The syntax error is due to Python 3.0–3.2 not supporting explicit unicode literals which were re-added to Python 3.3 with PEP 0414.

[cpburnz]

@aanand The Python 3.2 syntax error has been fixed and pathspec 0.3.4 has been released.

[aanand]

OK, after some investigation it turns out that pathspec isn't right for us. .dockerignore's syntax and semantics are dramatically different from .gitignores.

I've written a fairly comprehensive test suite that covers the correct behaviour (which I confirmed was correct by testing it against actual docker build invocations), and updated the code accordingly. There are no new external dependencies.

Here's what's new:

• Support all basic pattern forms: file, directory, *, ?, !
• Fix handling of wildcard patterns and subdirectories - */a.py should match foo/a.py, but not foo/bar/a.py
• Fix handling of directory patterns with a trailing slash - make sure they're handled equivalently to those without one (.dockerignore ignores trailing slash on directories #581)
• Fix handling of custom Dockerfiles - make sure they go in the tarball (dockerignore implementation assumes default Dockerfile name #730)

This is ready for review.

[aanand]

@cpburnz thanks for responding, by the way, and sorry we didn't end up using your library in the end!

[aanand]

Fixed the build.

[shin-]

LGTM - thanks!