Construct the absolute resource basePath from relative one, using the Api basePath

This change allows using relative paths (like "api/v0", "v0" or simply "/") for the Resource's basePath value. In such cases the absolute resource's basePath is being constructed from relative one, using the runtime value of the parent api basePath.
Examples: 
runtime api.basePath = http://myhost.com:8090/mywebapp/v0/api-docs
relative resource basePath = v0
=> absolute resource basePath =  http://myhost.com:8090/mywebapp/v0

This also addresses and resolves the issue #52.