WIP: Using OSX Keychain as backend

[moul]

Waiting for #10 for total rewrite, this is a working proof-of-concept

Speed is bad but we can think about using Keychain and .gauth file, so the user can choose between speed and improved-security.

cc @golimpio

[moul]

#10 (comment)

it looks good to me.
I ran a few tests and it worked fine.
It's a bit slower than the config file (like you said), but since it runs a command line for each key, it's expected to be slower any way.
May be using the Using Python Keyring Lib would improve the performance, but it can be investigated for a next version, I think.

👍

[moul]

Too slow, we need to cache the list of keys and only call the slow resolver when asking specifically for a key