cross platform support

[mr-tron]

for linux enough change
[DllImport("libcrypto.dll", CallingConvention = CallingConvention.Cdecl)] to
[DllImport("/usr/lib/x86_64-linux-gnu/libssl.so", CallingConvention = CallingConvention.Cdecl)]

[MihaZupan]

I can make the path be a constant string you can change in one place, but I don't know that there's a fixed location for openssl libraries on linux.

[mr-tron]

I think you can just add libssl.so to repo, like libcrypto.dll