Convert Stream class to new Readable/Writable interfaces

[nozavroni]

• [x] Need to convert the CSVelte\Input\Stream and CSVelte\Output\Stream classes to the new CSVelte\Contract\Readable and CSVelte\Contract\Writable interfaces.
• [x] Write a new CSVelte\IO\Stream class (with converted classes from above)
  • [x] Need to write unit tests for stream context options (http://php.net/manual/en/function.stream-context-create.php)
• [x] Delete CSVelte\Input\Stream and CSVelte\Output\Stream
• [x] Implement getContents() method (and test)
• [x] Add __toString() method to IO classes
• [x] Add remaining StreamInterface methods from issue #107
• [x] Update documentation to reflect these changes

[nozavroni]

Check this out... I like it better than what I'm doing currently... Very clever...

https://github.com/binsoul/io-stream/blob/master/src/AccessMode.php

[nozavroni]

Implemented and tested, although I need a new ticket for that most recent comment.