Develop

Minor changes and refactoring; mainly to workout pull-request workflow (is it good medium to communicate; will we use code reviers and so one)

## Examples Fixup

Minor bug introduced with `skel:do/2` function, where in examples receive clause was removed, but function call was not changed from `run/2` to `do/2`.

Renamed unused variables to omit compilation warnings.

## Unused header

Removed empty header file and all directives including him.

## Refactoring

Simplified call and return value form `sk_source:make/0` and `sk_sink:make/0`.  Now function calls seem cleaner. Added `make/1` variants if receivers Pid would have to be passed distinctly.

## Unit test

For now close to usage examples. Not testing internal logic of skeleton, but at least running the code.