Testgres

Small fix: `PostgresNode.safe_psql()` now raises error on wrong input (issue 52).

Small fix: make `NodeBackup` aware of inheritors of `PostgresNode`.

* Add `suppress` option to `PostgresNode.poll_query_until()` (PR 51);
* Add `format` option to `PostgresNode.dump()` (PR 32);
* Add `promote()` method to `PostgresNode` (PR 47);
* Add `version` property to `PostgresNode` (PR 49);
* Add support for logical replication (PR 42);

* Add `wait` option to `PostgresNode.start()` (PR 45);
* `PostgresNode.reload()` now returns `self`;
* Add support for replication slots (PR 41);

* New dependency: `psutil`;
* New properties:
- `child_processes` - list of `ProcessProxy` objects representing PostgreSQL processes,
- `auxiliary_processes` - subset of `child_processes`, represents auxiliary processes (e.g. `autovacuum`),
- `auxiliary_pids` - dict of `{ ProcessType : PID }` pairs,
- `source_walsender` - available on replica: represents master's `walsender`;
* New context manager `scoped_config()`: temporary change `GlobalConfig`;

* Testgres is able to provide unique system ids for cached instances (see 35);
* Temp dirs use `tgs_` as prefix;

* Improved `pgbench` API (see `pgbench_run()` etc);
* Set `allow_streaming=True` in `init()` and `default_conf()` by default;
* New option `unix_sockets=True` in `init()` and `default_conf()`;
* Fixed `base_dir` ignore if provided `base_dir` does not exist (31);
* `StartNodeException` now includes last N lines of `postgresql.log`;
* Improved exception handling (using `six.raise_from`);
* Auto name generation for `PostgresNode` (see [`generate_app_name`](https://github.com/postgrespro/testgres/blob/1.5.0/testgres/utils.py#L60));
* Provide compatibility mode for `execute()`, `psql()`, `safe_psql()` and `poll_query_until()` methods (see [`positional_args_hack`](https://github.com/postgrespro/testgres/blob/1.5.0/testgres/utils.py#L244));