implement trivial opengeometadata route to send metadata files

rom a cloned repo in `/var/cache/opengeometadata`. related to geoblacklight/geoblacklight#104
This is a trivial implementation, but my hope was to have the following REST paths:

```
/opengeometadata/layers/edu.stanford.purl/yn681zp0153 (returns default metadata as XML)
/opengeometadata/layers/edu.stanford.purl/yn681zp0153.xml (same as above)
/opengeometadata/layers/edu.stanford.purl/yn681zp0153.html (returns default metadata as HTML)
/opengeometadata/layers/edu.stanford.purl/yn681zp0153/mods (returns MODS metadata as XML)
/opengeometadata/layers/edu.stanford.purl/yn681zp0153/mods.html (returns MODS metadata as HTML)
```
Then, the .xml formats can have PUT requests in the distant future. I'm not sure whether the parameters would work for a PUT correctly. Maybe an alternative is to always require a metadata format and the default would just be `default`

```
/opengeometadata/layers/edu.stanford.purl/yn681zp0153/default.xml
```

that probably makes most sense.