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

先從所參照的 libgit2 CMakeLists.txt 屬性和連結的 MonoDevelop build_libgit2.sh 指令碼開始，接著整理 issue 中關於 Windows、Mac 和 Linux 建置的說明。完成的標準是文件說明必要的相依性、平台特定的指令、輸出函式庫，以及 libgit2 使用 libssh2 所需的參數。