Pypyr

- update ruamel.yaml to >0.15. This is a major, interface breaking change on the part of ruamel. pypyr consumers are not affected, however.
- add fileformatyaml.yaml example

- let twine choose pypi url rather than specify it
- minor README updates for pype recursion
- no end-user change in this patch

- pypyr.steps.pype - run nested pipelines, run another pipeline from a pipeline step
- includes refactor of the pypyr core to accept already existing context
- the pypyr `Context` has a new `working_dir` attribute
- update api docstring documentation for pipelinerunner.py
- README updates for the new functionality

- contextsetf step. Set context keys from formatting expressions with {substitutions}.
- assert step - friendlier error messages include type information
- README updates for new functionality

- add pypyr.steps.assert. Provide debug .assert style functionality in a pypyr step
- README updates
- assert
- nested formatting expressions
- thank yous

- tar archive would take an inpath like */path/to/mydir* and tar the entire path as */path/to/mydir*, rather than just add *mydir* to the tar. As of now, only the destination dir, and not its parents, will end up in the tar.
- pin ruyaml dependency to <0.15. There will be API changes in the 0.15+ versions, that might lead to warnings that your users could see.