Added SVG file for the main icon to the vendor directory.

It may come handy to get the icon at different resolutions with minimal degradation in the future.
Recreated the 18x18 px version of the main icon from the full-sized SVG file.

It looks better than the previous version, which was a resized PNG file.
Renamed "spec_helper.rb" to "rails_helper.rb" as a preliminary to follow the rspec-rails 3 upgrade procedure:

https://github.com/rspec/rspec-rails#upgrading-an-existing-app