Add the source map generator

Based on magic comments from the elia/source-map branch on opal:
- /*file:path.js.rb*/ for files
- /*:34*/ for lines
Tell bundler where to get the source-maps enabled opal
Cleanup test_app routes
Serve source-maps from a controller
Add a basic spec for source-maps
Pathnames ain't strings
Provide the test_app with a root path
Use `rails new -j opal` in the test app
Add the pathname stub to the processor spec
Ignore tmp dir
301 opal/source_map has moved permanently to opal
Don't require everything
Cleanup tmp/cache/assets before each app run
Fix branch name
We need latest opal-jquery to run
Merge branch 'source-maps'