Allow setting attributes when instantiating objects

[henare]

# Now:
writeitinstance = WriteItInstance.new
writeitinstance.base_url = ENV["WRITEIT_BASE_URL"]
writeitinstance.url = ENV["WRITEIT_URL"]
writeitinstance.username = ENV["WRITEIT_USERNAME"]
writeitinstance.api_key = ENV["WRITEIT_API_KEY"]

# What it would be nice to be able to do:
writeitinstance = WriteItInstance.new(base_url: ENV["WRITEIT_BASE_URL"],
                                      url: ENV["WRITEIT_URL"],
                                      username: ENV["WRITEIT_USERNAME"],
                                      api_key: ENV["WRITEIT_API_KEY"])