Mirdata

* Changes top level API - datasets are now loaded with `mirdata.dataset('orchset')`
* Adds a Dataset object
* Renames `load` --> `load_tracks`
* Adds `dataset.choice_track()`
* Moves `Track` and `MultiTrack` base classes from `mirdata.track` to `mirdata.core`
* removes all `DATASET_DIR` variables and sets default download folder to the module name
* `Track` object no longer accept `data_home=None` - defaults are set in the `Dataset` object
* Fixes bug in beatport_key (311 )

**New Loaders**

- AcousticBrainz Genre
- cante100
- IRMAS
- Saraga Carnatic
- Saraga Hindustani
- Tonality classicalDB

**Features**

- Changes top level API - datasets are now loaded with mirdata.initialize('orchset')
- Adds a Dataset object
- Renames load --> load_tracks
- Adds dataset.choice_track()
- Moves Track and MultiTrack base classes from mirdata.track to mirdata.core
- Extends index structure beyond tracks and adds version as mandatory top-level key
- Introduces of datasets submodule
- Introduces of Annotation classes for different time-series data types
- Improve documentation
- Introduces support for remote indexes for big datasets
- Specify license in each dataset
- Config flags for automatic full-dataset testing each week (337)

**Bug fixes**

- 79 RWC beat parsing
- 311 Beatport Key fix index
- 326 Maestro downloading twice breaks
- 328 RWC metadata not parsing
- 329 Download message was not showing path
- 341 Support datasets with no metadata
- 346 Mridangam fix
- 347 Orchset unzip error
- 348 and 306 file download encoding utf8
- 351 Tinysol test_full_dataset fix
- 357 Cante100 partial download fix
- 374 adding bibtex to AcousticBrainz

**New Loaders**
- Saraga (no multitrack)
- IRMAS
- cante100
- Saraga Carnatic
- Saraga Hindustani
- AcousticBrainz Genre
- Tonality classicalDB

**Features**
- Updated Dataset object to be extended by every loader
- Switched annotation datatypes to dedicated classes from NamedTuples
- updated index structure to support more than tracks
- support for remote indexes
- automatic full-dataset testing each week (337)
- Improved documentation

**Bugfixes**
- Supports ziplib on windows (fixes 306)
- Fixed download_info printing issue (fixes 333)
- RWC metadata (338)
- validation for datasets without metadata (342)

- fixes bug in orchset download
- adds loader for Mridangam Stroke dataset

Adds a new MultiTrack base class