Sia v0.4.1

This is the first minor release for v0.4. It contains mostly bugfixes; nothing major in terms of functionality.

• You can now advertise a custom address with siac host announce mydomain.com. This solves the "dynamic IP problem" where hosts would advertise a dynamic IP address only for it to change soon after.
• The gateway now prunes its node list more aggressively. This should improve the overall health of the network (e.g. you should form new peer connections more quickly).
• The siag tool has been removed; no further updates to it are necessary. If you still need to use it, you can download binaries here.

I'd like to give a big shout out to @bitspill and @mtlynch, who contributed to this release! I can't tell you how cool it is to see members of the community pitching in to help us develop Sia. If you'd like to get involved yourself, check out our Issues page, and the #dev channel on our Slack.