Object management

This adds a higher level interface to the RestClient for managing Subnet & Host allocations. Specifically:

1. Adds the Netdot::Host and Netdot::Subnet classes.
2. Re-factors the Gem slightly to add the necessary boilerplate (E.g. lib/netdot.rb)
3. Re-factors most of the RSpec tests and adds tests for the Host & Subnet classes
4. Adds a Netdot.logger object for internal debug messages; by default this uses a NullLogger but applications can set it to any Logger object if they want to enable output from the netdot-restclient Gem.