Fix calling the path for calling PuppetDB API

Previous to this commit, the Puppet::Client#request method incorrectly 
constructed the path to call the PuppetDB API.  This commit uses the value
returned from self.class.base_uri to construct it correctly.

Further, the REAME did not specify the need to call specify which endpoint to
call on the PuppetDB API.  This commit updates the README to reflect the
proper use of the PuppetDB::Client#request method