Improvement/static route api

Static Route API modified to include preference in createStaticRoute method, and also with a improved REST API.

New objects had to be created in order to return the static routes information to the client. They have been included in the router.capabilities.api bundle.

This patch resolve following issues:

http://jira.i2cat.net/browse/OPENNAAS-1327
http://jira.i2cat.net/browse/OPENNAAS-1379