Proper module/import, so allow installing via `go get` now; More intensive testing

• Adopting proper usage of modules and importing, so now users can install Rediseen easily via go get as a new convenient installation option.
• Add go version 1.12 to 1.15 in the matrix of all tests, so the testing becomes more robust
• Fix an issue due to a minor change in Go 1.15 ("New warning for string(x)", https://golang.org/doc/go1.15)
• Other a few trivial changes