Make keyring dependency optional. Resolve #343.
Keyring now requires dbus-python so some action on this was needed. We
could just add another install requirement, but for a couple reasons I
don't suggest that approach:
- There is a reason secretservice doesn't package it automatically --
using system dbus-python can be a more stable experience. This way
gives people the flexibility to use system dbus-python if they choose.
- To @dmitche's point, this dependency doesn't necessarily make sense
for every bugwarrior use case:
> I don't feel especially great about having to install freedesktop components on my small VPS just so I can run bugwarrior there (I sync bugs to taskwarrior there hourly, and then sync tasks around to my other systems).