Pyznap

Added
- pyznap can now pull data over ssh, i.e. you can now send form local to local, local to remote,
remote to local and remote to remote. Note that remote to remote is not direct, but via the local
machine.
- `pv` now outputs status once per minute when stdout is redirected (e.g. to a file).

Changed
- Rewrote local/remote 'zfs send' commands in a more uniform manner.

Fixed
- Enforce python>=3.5 in setup.py.

Fixed
- Removed `configparser` dependency.

Added
- pyznap now uses compression for sending over ssh. Current supported methods are `none`, `lzop`
(default), `lz4`, `gzip`, `pigz`, `bzip2` and `xz`. There is a new config option (e.g. `compress = none`)
and a new flag `-c` for `pyznap send`.
- `mbuffer` is now also used on the dest when sending over ssh.

Changed
- Rewrote how commands are executed over ssh: Implemented own SSH class, removed paramiko dependency.
- General code cleanup.

Fixed
- Send would fail on FreeBSD due to missing stream_size.

Added
- Catch KeyboardInterrupt exceptions.

Changed
- Code cleanup.

Changed
- Changed frequency of 'frequent' snapshots to 1 minute. Interval at which 'frequent' snapshots
are taken can be controlled by cronjob. This allows users to take snapshots at different intervals
(1min, 5min, 15min, ...).
- Code cleanup in process.py. No more overwriting of subprocess functions.

Fixed
- Fixed pv width to 100 chars.