code consistency for add command ...

- factor out common code to add dependency
- also support <package>=<version-specifier> depspecs in dub's
  interactive init
- more specific handling of exceptions around querying registry
- abort dub add when querying latest version fails
- replace self-written (and incomplete) recipe finder and loader
  with existing loadCwdPackage and use package's rawRecipe

support version specifications in new add command ...

- Also simplify test to just grep for addition, fetching and building
  dependencies is already tested elsewhere.

Fix travis ci

add command test