separate precompile tests from manifest tests
write failing test for AssetUrlHelper helpers
alias path_to_asset to asset_path with error checking
write failing test for debug mode
don't check for required assets in precompile array in debug mode
make non-javascript and non-stylesheet assets match filter
remove unneeded code causing precompile errors
don't check runtime errors for other tests
assets version doesn't depend on asset host iff it's a Proc
Merge pull request #140 from pomnikita/fix-assets-version

assets version doesn't depend on asset host if it's a Proc
Merge pull request #143 from dskang/js-css-path

Use `asset_path` with error checking for all calls to `path_to_asset`