Add :time_entry method for retrieving single Time Entry records

[gregmatthewcrossley]

This PR adds a new method to the Client class: :time_entry. As opposed to the existing :time_entries method, this allows for the retrieval of one single time entry record. The new time_entry method requires a single attribute, the time entry ID. For example:

client = Harvesting::Client.new
time_entry = client.time_entry(123456789)