Fix config.assets.manifest Rake task and Railtie

Currently, `config.assets.manifest` is not functioning as it should be. However, currently there no problem if `config.assets.manifest` is nil.

- Fix `assets:precompile` Rake task. The `Sprockets::Manifest` object used in the Rake task is separate from the one created in the Railtie.
- Fix `config.assets.compile = false` case. It is necessary to initialize the `Sprockets::Manifest` object with both the path to the assets, and the manifest path as parameters.
- In Railtie initializer, rename internal variable `manifest_path` to `manifest_assets_path` for clarity, and revert its definition to what it was before PR #122
- Add additional test cases related to config.assets.manifest option.

Proper functionality of the `config.assets.manifest` also requires this PR: https://github.com/sstephenson/sprockets/pull/560. (Again, there is no issue if `config.assets.manifest` is left nil)