Build packages for client and yum repositories

As discussed at Madrid workshop.

Fixes #16.
Fixes #17.
Something like:
```
/14.6.0/
/14.8.0/
/14.10.0/
/aquilon/el6/
/aquilon/el7/
/externals/el5/
/externals/el6/
/externals/el7/
```

Or the other way around?
```
/el5/externals/
/el6/aquilon
/el6/externals/
/el7/aquilon/
/el7/externals/
```