Add josm-atlas to standard JOSM plugins list.

[hallahan]

It will be nice to just download this plugin using the standard JOSM plugin list mechanism, rather than requiring the user to build it themselves.

[floscher]

The easiest way is probably to publish GitHub releases and put a link to that file into https://josm.openstreetmap.de/wiki/PluginsSource .

I could prepare a PR soon, that would set Travis CI up to publish a *.jar file to GitHub each time a git-tag is pushed to this repo (I did already set that up for several other JOSM plugins).

[hallahan]

Yes, that would be awesome! It would be very much appreciated!