Add external/bin directory
Remove NamesTests.c to fix compilation on windows

The names directory contains only constants for representing particular
element names, and doesn't need tests. Removing NamesTests.c for the
time being as it inadvertently re-defined the LibTests symbol, which
caused compilation to fail on windows.

The way we use integer values to identify elements in the DOM tree needs
to be re-thought anyway, as it doesn't scale or separate for different
file formats. For example there's no situation in which you would have
ODF and OOXML elements in the same XML document; there needs to be a
separate "namespace", to to speak, of integer tags for each filter, with
common ones like HTML included.
Remove duplicate linkage to files in core

The object files generated from core were being included twice in the
command line to the linker, which was causing a warning under Visual
C++. This was due to an extra (unnecessary) reference to
$<TARGET_OBJECTS:core> in DocFormats/CMakeLists.txt.
Add script to download dependencies for windows
Add script to extract windows dependencies

This script unzips the archive downloaded by fetch_downloads.sh and puts
the appropriate files in the bin, lib, and include directories.

Some of the library names have been slightly changed in CMakeLists.txt
to match the fact that these scripts use different binary distributions
of the libraries than were used previously.
Don't store bin/lib/include dirs in repository

These are only intended to contain relevant files from the downloaded
binary distributions, and excluding them from the repository makes it
simpler to blow them away and replace them with those created by the
script when necessary.

The unistd.h file that was present in the include does not necessary,
because it was only the for zconf.h, but zconf.h does not attempt to
include unistd.h if HAVE_UNISTD_H is not set, as is the case on Windows.