Support Sia backup storage

[Wernight]

*Sia* (http://sia.tech/get-started/) is relatively easy to setup for users but it seems to really shine for backup storage for now.

The UI flow is shown at https://blog.sia.tech/getting-started-with-private-decentralized-cloud-storage-c9565dc8c854, but there is *Sia Daemon*
 (http://sia.tech/apps/) (see API at https://blog.sia.tech/api-quickstart-guide-f1d160c05235).

Why Sia? One arguable reason is that it's a distributed system with copies on 3 machines. Yes it's very new and linked to a crypto currency, but the real No.1 reason is the cost: About $0.16 / TB / month (https://www.reddit.com/r/siacoin/comments/5vl5an/sia_storage_costs_016_per_tb_per_month/).

For a comparison:
 - $10 /TB/mo Google Cloud Nearline storage
 - $7 /TB/mo Google Cloud Cold storage
 - $12 /TB/mo S3 Infrequent storage
 - $4 /TB/mo S3 Galcier storage