SSH support

Okay so, I'm going to lay down the docs over this issue so I don't lose them:

Windows:

We need cmake, zlib-1.2.8 package and libssh2 repo cloned.

Having these side-by-side, inside libssh2 directory:

cd path/to/libssh2_repo ; mkdir build ; cd build ;
cmake .. -DBUILD_TESTING=ON -DBUILD_SHARED_LIBS=ON -DENABLE_ZLIB_COMPRESSION=ON -DZLIB_LIBRARY="....\zlib-1.2.8\lib\zdll.lib" -DZLIB_INCLUDE_DIR="....\zlib-1.2.8\include"

This will build libssh2 dll and .lib inside path/to/libssh2_repo/build/src/Debug

It will link against zlib and we'd have to use zlib-1.2.8/zlib1.dll file side-by-side with libssh2.dll and libgit2.dll

TODO: Update docs on what parameters we need to send libgit2 to pick up libssh2. We have to modify all these properties manually, so we set FOUND manually, and so do we set link and include directories.

Mac:

brew install libssh2
build cmake with DUSE_SSH
copy libssh2 to output directory and install_name_tool -id libssh2.dylib libssh2.dylib

Linux:
Magic script: https://github.com/mono/monodevelop/blob/master/main/src/addins/VersionControl/MonoDevelop.VersionControl.Git/build_libgit2.sh

Commencez par les propriétés du CMakeLists.txt de libgit2 référencé et le script build_libgit2.sh de MonoDevelop lié, puis organisez les notes de compilation pour Windows, Mac et Linux présentes dans l’issue. La tâche est terminée lorsque la documentation explique les dépendances requises, les commandes propres à chaque plateforme, les bibliothèques produites et les paramètres nécessaires pour que libgit2 utilise libssh2.