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.
utils: add -DWITH_DOC -DWITH_TEST in Preferences

We add the two options WITH_DOC and WITH_TEST in Preferences. Theses
options are used when the users selects the configure package mode.
vle: fix executable status return

The status of the executable is badly in the package mode, if now no vpz
provided. For example, the following commands failed (even if rclean
success):

	$ vle-1.1 -P vle.output rclean
manager: fix logOptions structure

Remove the last comma of the structure.
An error occurs  during compilation.
template: fix the cmake script

Replace into VleCheckAndDeclareGeneratedModelsConfig.cmake
the lokk for differentialEquation by differential-equation