Dataset: use gem ancestry to manage dataset trees
Vps.Snapshot.Show: provide resolved URL parameters
Basic implementation of VPS subdatasets

Experimental support for dataset creation, listing and show.
It is possible to set custom mountpoint on creation of a dataset.
Dataset: cache full_name in the database
VPS subdatasets: fix mountpoints for nested datasets

It now behaves exactly how ZFS does. Child datasets are mounted
below its parent, wherever that is.
Resource Vps.Dataset: show mountpoint
Chain Dataset::Create: call hooks for DatasetInPool create
Vps.Dataset: move dataset path resolving to model
Gemfile: add mysql2
VPS dataset destroy, improve create, expiration

When destroying VPS subdataset, it is deleted from hypervisor only.
Backup dataset is kept, but its expiration is set to now+30 days.
Some cleanup regular task that will take care of backups will
be implemented later.

Subdataset creation now checks if created dataset doesn't already
exist on a backup pool and uses it if it does, removing
its expiration.