Sparkly

* Add instant iterative development mode. `sparkly-testing --help` for more details.
* Use in-memory db for Hive Metastore in `SparklyTest` (faster tests).
* `spark.sql.shuffle.partitions = 4` for `SparklyTest` (faster tests).
* `spark.sql.warehouse.dir = <random tmp dir>` for `SparklyTest` (no side effects)

* Fix: remove backtick quoting from catalog utils to ease work with different databases.

* Add ability to specify custom maven repositories.

* Make it possible to override default value of spark.sql.catalogImplementation

* Add KafkaWatcher to facilitate testing of writing to Kafka
* Fix a few minor pyflakes warnings and typos

* Fix: 40 write_ext.kafka ignores errors.