Add capability to create coverage store with external files

I added two new public functions to add coverage store pointing to external data file (ie: not uploading the file to the GeoServer):

```
def create_coveragestore_external_geotiff(...)
def create_coveragestore_external_mosaic(...)
```
I've also fixed a problem where an UnsavedCoverageStore object would not be saved under the specified workspace, but rather the default workspace only (tested with GeoServer 2.4.2).