Manage images with Assets module

[SimonLab]

To add assets to the application (e.g. images) we can create a Asset.elm module which will take the responsibility to expose the correct images' src in the application.
Having this single source of truth make it easier to change the names of the assets without having to change it on all the application.

An example of this Assets module can be found here: https://github.com/rtfeldman/elm-spa-example/blob/master/src/Asset.elm

• Add the dwyl logo to the application using the Asset module

[SimonLab]

see PR #5