Made library compatible with both Unix and Windows versions of gpg-agent

[mhenderson-so]

Honestly this took me about an hour to do this morning, so it might need a bit more looking over. But essentially this adds support for Windows gpg-agent into this library.

I added the support by using compile-time flags in gpgagent_unix.go and gpgagent_windows.go so that a different version of NewGpgAgentConn gets compiled in depending on which platform you compile with.

Because Windows does not have Unix sockets, and gpg-agent doesn't use named pipes (rather it binds a localhost TCP socket), the Windows version:

• Discovers the path to the socket file (environment variables are not reliable on Windows)
• Opens the socket file
• Reads the port number and the nonce out of the socket file
• Initiates a TCP connection instead of a Unix connection to 127.0.0.1:port
• Writes the nonce to the port
• Returns the TCP socket

I've only done limited testing, but it works great in the examples I've tried.

[jcmdev0]

Might be worth separating the parsing for some unit tests, but for the sake of advancing the code I forked out of another project for isolation, this is great =).

Thanks for the contribution!