IGearClient.add accepts environment variables - fixes 568

Fixes #568 
It looks like this code does result in a lot of lingering environment files, but gear doesn't seem to provide any way to remove them.
```
$ sudo find /var/lib/containers/env/contents/ -type f -exec sh -c "echo {}; cat {}; echo '----'" \;
...
/var/lib/containers/env/contents/97/976729112297
key2=value2
key1=value1
----
/var/lib/containers/env/contents/85/854195619613
key2=value2
key1=value1
----
/var/lib/containers/env/contents/17/175479029301
key2=value2
key1=value1
----
```

There's a `gear clean` command but it doesn't have any routines for cleaning up unreferenced environment files.