Add assets precompile paths of images and fonts in Rails Engine

This PR fixes #96.

> Currently, rails-assets places font files under vendor/assets/fonts/.
>
> However, in Rails 4 asset precompile, by default, only non-js/css files under app/assets/ are included [1]. Font files under vendor/assets/fonts are not included by default.

Similarly, image files are placed under /vendor/assets/images/.

For example, according to bootstrap-sass, this problem is resolved by adding paths in the Engine.
https://github.com/twbs/bootstrap-sass/blob/master/lib/bootstrap-sass/engine.rb
#96 also made a similar proposal.

Therefore, I've implemented in this way. There may be objections to this matcher of paths.