Small cleanup
Update gemspec

- Update specfile structure
- Bump sprockets dependency to v2.10+
- Disable gaurd notifications
Improve configuration options

Rename:

- `compress.js` -> `js_compressor`
- `compress.css` -> `css_compressor`
- `cache_assets` -> `cache`

Improve:

- `cache` can be specified as absolute or relative path. When specified as
  boolean, default path is used.
Reset cache upon config change
Update HISTORY
Version bump to v0.5.4