Clean gem structure

This commit moves files into lib/capistrano-new-relic and adds a version.rb file
The Gemspec is updated and dependencies added. Unused Gemfile is removed.
Refactor code to conform closer to Ruby best practices.

Cosmetic changes such as string quotes and concatination,
but also refactors away the `_cset` method,
as "private" methods on the global object is a code-smell.