Revisit godog suite compilation and build tools to support vendoring

[l3pp4rd]

should close #35

this is still in progress.
In general it will use standard go test, compile and link tools to build godog test suite.
Suite context will need to be exported same as Tests in go as usual.

• update screenshots based on related changes
• optimize builder.go and make sure vendoring support works as expected
• support for multiple GOPATHs
• test on windows
• try to support older than 1.5 go versions
• allow godog to run even in empty directory, with or without any feature file
• update doc blocks

Already functional, if someone wishes to use it.