Add support for a custom assets prefix
Reset prefix setting it to nil
Fix singleton R2AP::ViewHelpers.asset_path

Was relying on the singleton version of R2AP::ViewHelpers.assets_prefix
Add prefix examples for tasks & view_helpers specs
Let R2AP.config_ru respect the prefix
fix specs
update rspec
update rake
update sprockets and fix errors
support lower versions of sprockets
v0.3.0