Replace StashNotifier class with Module

Split top-level StashNotifier into BuildResult and HTTPClient classes to
support separation of concerns.
Style refactorings {} -> do...end
Completed refactor into module. All tests pass.

Split main http client functionality into separate class.

Now we have a StashNotifier module which defines a 'new' method which
instantiates a StashNotifier::BuildResult instance to get the
JSON-formatted build result, then instantiates a new HTTPClient to talk
to the Stash API.
Remove binding.pry breakpoints
Switch to single quotes in gemspec per Rubocop
Release v2.0.0

Add stash_pwd option to notify
Add VCR support for offline testing
Refactor StashNotifier into Module
  Create StashNotifier::BuildResult class
  Create StashNotifier::HTTPClient class

Fixes #8: Add to_s method to StashNotifier class