NEW reset catalog function

* Implements REST `reset` function taking `reload` as example
* Equivalent to request of REST API docs:
http://docs.geoserver.org/latest/en/user/rest/api/reload.html
IMP LayerGroups and workspaces

* Now we can use workspace parameter in `get_layergroups` and `get_layer` to
  return layergroups in a workspace.
* `workspace` attribute may be set to create a layergroup
* `workspace` attribute is now readable in a LayerGroup object
FIX Sets workspace from reposnse in get_layergroup
FIX Sets workspaces in get_layergroups
FIX Allow projection and projection_policy update

* The writers dict was not defined properly
Merge branch 'FIX_WMSlayer_srs_save' into gisce_version
Merge branch 'NEW_catalog_reset' into gisce_version
Merge branch 'NEW_layergroup_workspace' into gisce_version