Added description how to use config system, fixed missing copyright headers, improved example and isort CI (#125)

* Added description how to use config system related to #20.
* Added missing copyright headers
* Improve registry example to be cleaner
* Adde habitat_sim as first party lib to isort CI setup

Updated baselines readme, fixed links, added link to checkpoints.

- add config option NUM_SAMPLE_EPISODE, with default value -1 that
indicates use all episodes. When set value to non-negative integer,
pointna_dataset will sample the specified number of episodes.
- fix the functionality of merging multiple config files. Now can specify
more than one config file like this
	--task-config "a.yaml" "b.yaml"

* Update main README to properly install gym

* Update baseline README for gym install

* add extras_require in setup.py

* add exclude in setup.py for baselines

* add pytest as requirement

* Implement accessing setup() params within custom commands

Renamed baselines folder to habitat_baselines

Config system improvements: compositional config, added scenes_dir, removed config_dir, fixed eqa test (#77)

* Added scenes_dir parameter to Config to have no requirement of data symlink or data directory in working directory. scenes_dir will be used as prefix for scene_id during episodes dataset loading.
* Added overwriting options argument to get_config that unblocks parameter sweeping and experimentation capabilities from command line
* config_file path can be comma separated list of configs that will be merged in exact order. That will help to use separate configs for dataset specification and sensors setups. String with commas was chosen because that make it easy to pass list of configs as script argument.
* To make habitat-api easier to as package we decided to get rid of forcing loading of configs from config_dir param
* fixed unit test caused by latest habitat-sim update
* Added support of config list, corrected scene_path, added overwriting options, added EQA dataset changes, adopted quaternion suggestion
* Fixed headers