#4 first vis plugin

Hi all,

so this is my first, infancy Galaxy visualization plugin as discussed in #4.

1. I don't know whether it is a  good idea to commit the "binary" (compressed,minified) MSA viewer code into this repo. (There exists also a CDN version.)

2. Is there a better way to load a resource relatively to the plugin folder?
(this is seems a bit inflexible)
```
${h.javascript_link( root + 'plugins/visualizations/biojs-msa/static/msa.min.js' )}
```

3. Any other suggestions are welcome