Pystreamer

[Impl break] `Zipper` operator flips the meaning of its boolean flag
[Feature] Add `constant_of` `iterate` `generate` stream generators
[Feature] Add set cartesian product family generators
[Feature] Add text buffer split family generators

[Fix] bugs of stream operators processing incorrectly on `None` elements
[Feature] `cross` operation
[Feature] adjacent element pair operation family
[Feature] `zip` family operations
[Feature] collapse family operations
[Feature] `has` family operation
[Feature] adjacent group operations
[Feature] group by operations

[Feature] add `sorted`, `peek`, `intersperse`, element-based filter functions, `reduce_right`
[Feature] add count / key options to distinct operation

[Break] Remove the support of `Dragon` as an alias of `Stream`
[Feature] Add collector definition and collector for counting
[Feature] Add commonly used stream consumer functions - now very similar to `Stream` of Java

[Break] Remove the support of python 2
[Break] Change function names of `streamer.util`
[Improvements] Add some small functionalities. More to come.

Stable release - prepared for conda-forge