Rnc

Quickfix: stream handler level was set to `WARNING` instead of `NOTSET`.

Changed
* Docs fixed and improved.
* Way to make async request to RNC was changed.
* Now Python3.7 is required.
* Required libraries were fixed, lxml and new aiojobs were added.
* Making a folder for csv files and media files by default was removed.
Folder will be created when `.dump()` or `.download()` method is called.
* Compare operators were removed from `Corpus` objects.
* `subcorpus['en']` and `subcorpus.en`, `subcorpus['Pushkin']` and
`subcorpus.Pushkin` now available for subcorpus.
* More logging messages were added.
* Downloading media files in `MultimodalCorpus` was fixed.
* Logger is made in `__init__` once and used as one in all modules.

Other minor fixes and improvements.
Added
* Chinese parallel corpus added.

Changed
* Logging/creating a logger simplified.

Added
* Additional info from the first RNC page.
* `MultimodalCorpus`.

Changed
* Examples improved and fixed.
* Docs fixed.
* Setting loggers/handlers levels.

Added
* Some features to Example.

Minor fixes and improvements.

Changed
* Docs changed, extended.
* Corpora were renamed.

Added
* MultilingualParaCorpus.
* TutoringCorpus.

Fixed
* Some Corpora inherited from MainCorpus didn't work.

Minor fixes and improvements.

Changed
* Default filename contains letters and digits, len = 8.

Added
* Corpora classes to `rnc.`
* Working with file: validating, that the Corpus type in the file
is equal to the Corpus class type.
* Requesting two or more words with one str – Issue 5 closed.
* Deleting the example by the index – Issue 6 closed.
* Method `filter` to Corpus – Issue 9 closed.

Fixed
* Distance between words set – Issue 7 closed.
* Order of texts in the ParallelCorpus – Issue 8 closed.

Other minor fixes and improvements.