win32: fix PathWin32::getTempFile

Fix the confusion between the directory name and the filename
(Closes #55)
utils: use Boost::Asio rather than libcurl

DownloadManager now relies on Boost::Asio to perform a HTTP GET request
for downloading files (Closes #56).
vle: remove useless dependencies

The Boost::Asio does not depend to openssl libssh2 libssl libidn gnutls
hogweed libcrypto nettle. So we remove them.
vle: restore the libxml2 initialization

We restore the xmlInitParser() in vle::Init constructor.
utils: add unit test for DownloadManager

We add a test in DownloadManager. This unit test check the download of
the vle-1.0.0.dtd from www.vle-project.org server. We use BOOST_CHECK
instead of BOOST_REQUIRE.
utils: export DownloadManager symbols

To use DownloadManager class from external prorgam (as unit test), we
need to export the symbols.
utils: fix coding style in DownloadManager

Fix several coding style errors.