Merge pull request #66 from pmienk/master

Removal of `package_dependency`, rationalization with `use`
Problem: No model to represent the api of a class
Solution: Add zproject_class_api.gsl
Merge with origin/master
Merge pull request #67 from jemc/master

Problem: No model to represent the api of a class
Problem: no license / contribution policy in README

Solution: added these.
Rebuilt README.md
Merge pull request #68 from hintjens/master

Problem: no license / contribution policy in README
Problem: Great article about the origin of zproject is not linked
Solution: Use the summary of hintens blog #79 and link to it for anyone
who likes a little background about zproject.
Merge pull request #69 from sappo/master

Problem: Great article about the origin of zproject is not linked
Remove repetition, rely on default PKG_CHECK_MODULES behavior augmented with follow on conditionality.