Merge pull request #3 from elia/cleaner-config-ru

Use a straighter syntax inside config.ru
bump
v0.2.0
thanks Elia
Add support for a custom assets prefix
Move development deps to gemspec (from Gemfile)
Reset prefix setting it to nil
Fix singleton R2AP::ViewHelpers.asset_path

Was relying on the singleton version of R2AP::ViewHelpers.assets_prefix
Update RSpec and add good config defaults
Add prefix examples for tasks & view_helpers specs
Let R2AP.config_ru respect the prefix