Merge pull request #4 from malat/master

Update NFluent library
Adds a png banner
Fixes the png banner hyperlink.
Adds introduction to the Readme.md
Update README.md
Update README.md
Update README.md
Update README.md
Update README.md
Integrates the CONTRIBUTING.md files.
Migrates to the new extensibility model of NFluent, based on method extensions and a new covariant IFluentAssertion<out T> interface.
Migrates to the new extensibility model of NFluent, based on method extensions and a new covariant IFluentAssertion<out T> interface.
Migrates to the new version of NFluent (based on extension methods).
Git ignores .orig files.